Summary of Significant Accounting Policies (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Earnings Per Share
Preferred stock, shares outstanding	12,050		12,275
Net income loss available to common stockholders
Net income	$ 65,271,000	$ 78,619,000
Net income applicable to preferred stock	12,358,000	15,272,000
Net income applicable to common stock	52,913,000	63,347,000
Determination of shares:
Weighted-average common shares outstanding (in shares)	77,553,000	75,994,000
Assumed conversion of dilutive employee stock-based awards (in shares)	2,940,000	2,222,000
Assumed conversion of restricted stock (in shares)	99,000	138,000
Assumed conversion of preferred stock (in shares)	22,295,000	27,278,000
Diluted weighted-average common shares outstanding (in shares)	102,887,000	105,632,000
Anti-dilutive securities, options to purchase shares outstanding	235,125	3,122,903
Calculation of basic EPS:
Net income applicable to common stock	52,913,000	63,347,000
Weighted-average common shares outstanding (in shares)	77,553,000	75,994,000
Basic earnings per common share	$ 0.68	$ 0.83
Calculation of diluted EPS:
Net income (loss)	65,271,000	78,619,000
Diluted weighted-average common shares outstanding (in shares)	102,887,000	105,632,000
Diluted earnings per common share	$ 0.63	$ 0.74
Common stock price less than $45
Determination of shares:
Common stock price (in dollars per share)	$ 45
Increase in diluted weighted-average common shares outstanding	26,777,778
Common stock price between $45 and $67
Determination of shares:
Aggregate liquidation preference of preferred stock	$ 1,205,000,000
Common stock price between $45 and $67 | Maximum
Determination of shares:
Common stock price (in dollars per share)	$ 67
Increase in diluted weighted-average common shares outstanding	26,777,778
Common stock price between $45 and $67 | Minimum
Determination of shares:
Common stock price (in dollars per share)	$ 45
Increase in diluted weighted-average common shares outstanding	17,985,075
Common stock price more than $67
Determination of shares:
Common stock price (in dollars per share)	$ 67
Increase in diluted weighted-average common shares outstanding	17,985,075