September 5th, 1998 was the last day the NFL record book was safe.

The following day, Indianapolis Colts rookie Peyton Manning began a nearly two-decade assault on the league’s passing records, kicking off an unparalleled 18-year career with the Colts and Broncos.

Manning, who announced his retirement Monday, owns the single-season and all-time records for yards and touchdowns, is a five-time league MVP, and seven-time First Team All-Pro. He also became the first quarterback in history to win a Super Bowl with two different franchises this February, winning Super Bowl 50 with Denver after winning Super Bowl XLI with the Colts.

And this is honestly just scratching the surface.

Manning is second to Brett Favre in terms of pass attempts with 9,380, so it makes sense he would hold all-time volume records such as yards and touchdowns. What sets Manning apart from players like Favre, though, is his efficiency, as he has also averaged 7.23 net yards per pass, which is more than any player.

His dominance also still holds up once we adjust for the fact that Manning has played in an era of inflated passing averages (we’ll get to that).

Manning has taken heat for his postseason play during his career, but this is more fiction than fact. It may have been true in 2002, but is no longer the case, as Manning has been one of the better postseason quarterbacks of his generation.

He came into this past postseason ranked third in the NFL since 1999 in postseason net yards per pass and fourth in adjusted net yards per pass, which factors in both touchdown and interception rate.

If you’re unconvinced, read what I wrote about Manning and Tom Brady on the eve of this past AFC Championship, then meet us back here.

The Manning Century

Our in-house efficiency metric, Net Expected Points (NEP), dates back to the 2000 season, and has essentially belonged to Manning.

He has had the top seasons in terms of both Passing NEP and NEP per drop back, and actually did so in different seasons.

In 2013, Manning set the NFL record for passing yards (5,477) and touchdowns (55), in addition to attempts and completions. He also posted 278.52 Passing NEP, marking the fifth time he posted one of the 12 best NEP seasons.

Manning complemented the volume with an insane level of efficiency, and led the league in both net yards per pass and Passing NEP per drop back that season.

This was actually not his best season on a per drop back level, though. That would be 2004, and there’s a case to be made that was the best season any quarterback has ever had.

The raw numbers of staggering enough: Manning completed 336 of 497 passes for a then-NFL record 49 touchdowns and 10 interceptions. His 9.9% touchdown per pass rate that season remains a post-merger record, and his 9.78 Adjusted Net Yards per Attempt is also the best ever.

If we adjust for era, Manning’s touchdown rate in 2004 relative to league average is the best ever by a long shot, coming in at over 4.5 standard deviations above the mean. His rate is a full standard deviation better than Brady’s in 2007 and Rodgers’ in 2007, which are tied for second.

In terms of ANY/A relative to league average, Manning’s 2004 is also on top, joining Dan Marino’s 1984 and Rodgers’ 2011 as the only seasons with an ANY/A three standard deviations above the mean.

Using Adjusted Yards per Attempt (which does not factor in sacks, but allows us to look at seasons before 1969 when we don’t have sack rate data), Manning’s 2004 comes in third, behind only Rodgers’ 2011 and Nick Foles’ (Chip Kelly fueled but still frankly inexplicable) 2013. Manning edges out Otto Graham’s 1953, Sid Luckman’s 1943, and Roger Staubach’s 1971.

Swinging things back around to NEP per pass, Manning averaged 0.45, and has been in the top 12 four times in his career.

Finally, there’s Success Rate, which is the percentage of plays which produce positive NEP. It’s an improvement over completion percentage, because it doesn't credit the quarterback for things like a one-yard dump-off on 3-and-10.

Manning actually does not own the single-season record here (shocking, I know), but he does have five top-12 seasons (and six top-13 seasons).

Best Ever

NEP is our preferred metric, but Adjusted Net Yards are a good proxy, and have the advantage of letting us go back to 1969. ANY/A is useful because it correlates well with both winning and future iterations of itself.

Adjusted yardage (which, again, discounts sacks) is inferior (sacks are important and quarterbacks generally have a good deal of control over them) but it does fill in the gaps for 1920 through 1968. When we look at both numbers relative to the league average (AY/A+ and ANY/A+), it allows us to make comparisons across eras.

AY/A+ and ANY/A+ are scaled so 100 is average and 15 is one standard deviation, and Manning has posted a 120 ANY/A+ for his career.

Among quarterbacks with at least 4,000 pass attempts, that trails only Steve Young, Joe Montana and Rodgers. What sets Manning apart from these legends, and essentially everyone else, though, is that Manning has maintained this level of efficiency for a considerably longer period of time.

Manning has thrown more than twice as many passes as Young and Rodgers, and nearly 4,000 more than Montana, and tops the group of 12 quarterbacks who have thrown at least 6,000 passes in their career in terms of ANY/A+ (Marino and Brady are the other players in this group of 12 who have been better than a standard deviation better than average).

When looking at AY/A+ (which, again, discounts sacks, but goes back to 1920), Manning had eight seasons with a AY/A+ greater of 120, the most of any player (Young is next with seven, while Dan Fouts, Rodgers, Ben Roethlisberger, and Johnny Unitas are tied with six).

AY/A, though, doesn't factor in one of Manning’s greatest skills: his quick release, which helped him post the fifth-best sack rate relative to league average, despite an offensive line than ranged from stellar to poor and most things in between.

Thanks in part to this, Manning has 11 seasons with an ANY/A+ greater than 120, which is almost twice as many as the players in second place.

Manning played at a level few quarterbacks ever reached, and did so for longer than anyone. For this sustained excellence, we should consider him the greatest quarterback of all time.