Summary of Significant Accounting Policies (Details 2) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Earnings Per Share
Preferred stock, shares outstanding	12,275		12,275		12,275
Net income loss available to common stockholders
Net income	$ 66,667,000	$ 75,989,000	$ 145,286,000	$ 127,517,000
Net income applicable to preferred stock	12,914,000	14,396,000	28,183,000	24,185,000
Net income applicable to common stock	53,753,000	61,593,000	117,103,000	103,332,000
Determination of shares:
Weighted-average common shares outstanding (in shares)	76,257,000	78,387,000	76,126,000	78,275,000
Assumed conversion of dilutive employee stock-based awards (in shares)	2,595,000	1,858,000	2,471,000	1,694,000
Assumed conversion of preferred stock (in shares)	27,278,000	27,278,000	27,278,000	27,278,000
Diluted weighted-average common shares outstanding (in shares)	106,130,000	107,523,000	105,875,000	107,247,000
Anti-dilutive securities, options to purchase shares outstanding	1,700,528	2,816,053	3,125,403	2,894,152
Calculation of basic EPS:
Net income applicable to common stock	53,753,000	61,593,000	117,103,000	103,332,000
Weighted-average common shares outstanding (in shares)	76,257,000	78,387,000	76,126,000	78,275,000
Basic earnings per common share	$ 0.70	$ 0.79	$ 1.54	$ 1.32
Calculation of diluted EPS:
Net income	66,667,000	75,989,000	145,286,000	127,517,000
Diluted weighted-average common shares outstanding (in shares)	106,130,000	107,523,000	105,875,000	107,247,000
Diluted earnings per common share	$ 0.63	$ 0.71	$ 1.37	$ 1.19
Common stock price less than $45
Determination of shares:
Common stock price (in dollars per share)	$ 45		$ 45
Increase in diluted weighted-average common shares outstanding			27,277,778
Common stock price between $45 and $67
Determination of shares:
Aggregate liquidation preference of preferred stock	$ 1,230,000,000		$ 1,230,000,000
Common stock price between $45 and $67 | Maximum
Determination of shares:
Common stock price (in dollars per share)	$ 67		$ 67
Increase in diluted weighted-average common shares outstanding			27,277,778
Common stock price between $45 and $67 | Minimum
Determination of shares:
Common stock price (in dollars per share)	$ 45		$ 45
Increase in diluted weighted-average common shares outstanding			18,320,896
Common stock price more than $67
Determination of shares:
Common stock price (in dollars per share)	$ 67		$ 67
Increase in diluted weighted-average common shares outstanding			18,320,896