Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended		3 Months Ended		6 Months Ended
	Mar. 31, 2012	Feb. 29, 2012	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Dilutive Impact of Common Stock Equivalents on Earning per Share (Abstract]
Net income			$ 183,060	$ 158,762	$ 366,953	$ 362,331
Less: Dividend requirements on preferred stock			0	(1,060)	0	(2,120)
Basic earnings per share: [Abstract]
Earnings available to common shareholders			183,060	157,702	366,953	360,211
Weighted average common shares outstanding - basic (in shares)			487,717,000	484,918,000	487,538,000	484,283,000
Earnings available to common shareholders - basic (in dollars per share)			$ 0.38	$ 0.33	$ 0.75	$ 0.74
Effect of dilutive securities: [Abstract]
401(k) equity awards (in shares)			300,000	323,000	468,000	492,000
Diluted earnings per share: [Abstract]
Earnings available to common shareholders			$ 183,060	$ 157,702	$ 366,953	$ 360,211
Weighted average common shares outstanding - diluted (in shares)			488,017,000	485,241,000	488,006,000	484,775,000
Earnings available to common shareholders - diluted (in dollars per share)			$ 0.38	$ 0.33	$ 0.75	$ 0.74
Antidilutive weighted-average options outstanding excluded from earnings per share calculation (in shares)			0	2,400,000	0	2,500,000
Shares Repurchase [Abstract]
Number of shares of common stock approved to be repurchased (in shares)		700,000
Number of shares of common stock repurchased (in shares)	700,000
Average purchase price per share of common stock repurchased (in dollars per share)	$ 26.42
Number of shares of common stock purchased for settlement of equity awards (in shares)		900,000