Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Dilutive Impact of Common Stock Equivalents on Earnings per Share (Abstract]
Net income	$ 196,857	$ 183,060	$ 433,427	$ 366,953
Basic earnings per share [Abstract]
Earnings available to common shareholders	196,857	183,060	433,427	366,953
Weighted average common shares outstanding - basic (in shares)	497,747	487,717	493,786	487,538
Earnings available to common shareholders - basic (in dollars per share)	$ 0.40	$ 0.38	$ 0.88	$ 0.75
Effect of dilutive securities [Abstract]
401(k) equity awards (in shares)	289	300	517	468
Diluted earnings per share [Abstract]
Earnings available to common shareholders	$ 196,857	$ 183,060	$ 433,427	$ 366,953
Weighted average common shares outstanding - diluted (in shares)	498,036	488,017	494,303	488,006
Earnings available to common shareholders - diluted (in dollars per share)	$ 0.40	$ 0.38	$ 0.88	$ 0.75