The more relevant number to use is Int% (Ints/Attempts). That number tells you the frequency with which they threw interceptions. Taking your same QBs and adding in a few more:
Favre - 3.3
Elway - 3.1
Montana - 2.6
Young - 2.6
Aikman - 3.0
Staubach - 3.7
Marino - 3.0
Unitas - 4.9
Starr - 4.4
Fouts - 4.3
Brady - 2.4
Manning -2.8
Blanda - 6.9
One thing you'll notice is over time the trend has been for the % to decrease. This is largely accounted for by the increasing sophistication of passing schemes (multiple 3+ receiver sets), and systems which emphasize shorter, higher percentage passes. Montana and Young really shine here because they outpace the trend. You'll also note that the next generation of elite QBs has that number under 3%. That number may be reaching a point where it's not going to get much lower. For those interested, Rodgers was at 2.4 this year. A very good number, but not yet in this discussion because it's based on only one year.
The next set of numbers I'll show are the top ten Intercepted QBs with their INT%:
Favre 310(3.3)
Blanda 277(6.9)
Hadl 268(5.7)
Testaverde 267(4.0)
Tarkenton 266(4.1)
Snead 257(5.9)
Unitas 253(4.9)
Marino 252(3.0)
Tittle 248(5.6)
Hart 247(4.9)
It becomes quite apparent that Favre isn't throwing INTs as often as any of the other men in the top ten save Marino, and his percentage is closer to Marino's than it is to the rest. Favre's total is because he has more attempts than anyone else ever. That's due to him not missing games, playing longer than most QBs do, and being asked to pass more than most QBs are. Frankly, the idea Favre is some kind of turnover machine is a myth, and it's not borne out by the data. (Interesting footnote, 5 of the top 10 are in the Hall of Fame, and Favre will likely be the 6th). Since 2000, Favre has been below his career INT% every year except 2003, 2005, and 2008.
Final comparison:
league average for the last 17 years 3.23 INT%
Favre 3.3%
league avg last 17 yrs TD% 3.97
Favre 5.0
Favre has been average at INT% compared to the rest of the league over his career. Where he has excelled has been in throwing TDs more often than the rest of the league.