Earnings Per Share (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Nov. 29, 2010	Aug. 31, 2010
Common Stock [Abstract]
Total number of shares of common stock issued to banking counterparty (in shares)													21,850,000
Initial pricing of equity forward instruments based on the common stock offering price, net of underwriting fees (in dollars per share)													$ 20.855
Offering price of common stock in public offering, before underwriting fees (in dollars per share)													$ 21.50
Underwriting fees per share of common stock in public offering (in dollars per share)													$ 0.645
Cash proceeds from settling forward agreements												$ 449,800,000
Common Stock Equivalents [Abstract]
Antidilutive weighted-average options outstanding excluded from earnings per share calculation (in shares)									2,100,000	5,400,000	7,600,000
Dilutive Impact of Common Stock Equivalents on Earning per Share (Abstract]
Net income	140,509,000	338,332,000	158,762,000	203,569,000	136,634,000	312,306,000	139,776,000	167,118,000	841,172,000	755,834,000	680,887,000
Less: Dividend requirements on preferred stock									(3,534,000)	(4,241,000)	(4,241,000)
Less: Premium on redemption of preferred stock									(3,260,000)	0	0
Basic earnings per share [Abstract]
Earnings available to common shareholders	140,509,000	333,658,000	157,702,000	202,509,000	135,573,000	311,246,000	138,716,000	166,058,000	834,378,000	751,593,000	676,646,000
Weighted average common shares outstanding - basic (in shares)									485,039,000	462,052,000	456,433,000
Earnings available to common shareholders - basic (in dollars per share)	$ 0.29	$ 0.69	$ 0.33	$ 0.42	$ 0.29	$ 0.68	$ 0.30	$ 0.36	$ 1.72	$ 1.63	$ 1.48
Effect of dilutive securities [Abstract]
Equity forward instruments (in shares)									0	700,000	0
401(k) equity awards (in shares)									576,000	639,000	705,000
Stock options (in shares)									0	0	1,000
Diluted earnings per share [Abstract]
Earnings available to common shareholders									$ 834,378,000	$ 751,593,000	$ 676,646,000
Weighted average common shares outstanding - diluted (in shares)									485,615,000	463,391,000	457,139,000
Earnings available to common shareholders - diluted (in dollars per share)	$ 0.29	$ 0.69	$ 0.33	$ 0.42	$ 0.29	$ 0.67	$ 0.30	$ 0.36	$ 1.72	$ 1.62	$ 1.48