Stockholders' Equity (Details) (USD $)	1 Months Ended											12 Months Ended
	Oct. 31, 2012	Jul. 31, 2012	Apr. 30, 2012	Mar. 31, 2012	Nov. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Jan. 31, 2011	Dec. 31, 2010	Oct. 31, 2010	May 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Spread Over LIBOR												43	25
Preferred Stock, Shares Authorized												10,000,000
Preferred stock, shares issued												6,900,000	0
Issuance of common stock, value												$ 3,601,000,000	$ 4,376,000,000	$ 1,056,000,000
Public offering of common stock		36,800,000		71,200,000	40,500,000	49,700,000	32,200,000	26,900,000	8,300,000	13,200,000	6,900,000	108,000,000	149,300,000	28,400,000
Preferred Stock, Par or Stated Value Per Share												$ 0.01	$ 0.01
Proceeds from Issuance of Preferred Stock and Preference Stock			167,000,000									167,000,000	0	0
Preferred Stock, Dividend Rate, Percentage			8.00%
Preferred Stock, Liquidation Preference Per Share			$ 25.00
Stock Repurchase Program, Authorized Amount	500,000,000
Stock Repurchased During Period, Shares												(2,700,000)
Stock Repurchased During Period, Price Per Share												$ 29.00
Stock Repurchased During Period, Value												(77,000,000)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period												7,000	4,500	3,000
Share-based Compensation												282,000	176,000	94,000
At the Market Offering Program [Member]
Public offering, average price per share												$ 31.41	$ 29.25	$ 29.13
Issuance of common stock, value												298,000,000	273,000,000	127,000,000
Public offering of common stock												9,500,000	9,400,000	4,400,000
Shares available for issuance												16,700,000
Dividend Reinvestment And Direct Stock Purchase Plan [Member]
Issuance of common stock, value													$ 15,000,000	$ 204,000,000
Public offering of common stock													500,000	7,700,000
Shares available for issuance												4,700,000
Issuance of Equity [Member]
Preferred Stock, Par or Stated Value Per Share			$ 24.21
Restricted Stock [Member] | Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period												12,000	12,000	4,500
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value									25.73			$ 29.48	$ 29.05	$ 25.73
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant												62,500
Restricted Stock [Member] | Each Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period												3,000	3,000	1,500