Stockholders' Equity (Details) (USD $)	1 Months Ended							3 Months Ended		9 Months Ended		12 Months Ended	3 Months Ended	9 Months Ended	109 Months Ended	1 Months Ended	98 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Director Quarter	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 DRSPP	Sep. 30, 2012 DRSPP	Sep. 30, 2012 DRSPP	Dec. 31, 2008 Controlled Equity Offering Program	Sep. 30, 2012 Controlled Equity Offering Program
Dividends on Preferred Stock
Preferred stock, shares issued	3,840,000			3,840,000				3,840,000		3,840,000		3,840,000
Preferred stock, shares outstanding	3,840,000			3,840,000				3,840,000		3,840,000		3,840,000
Preferred stock, par value (in dollars per share)	$ 0.01			$ 0.01				$ 0.01		$ 0.01		$ 0.01
Preferred stock, liquidation preference (in dollars per share)	$ 25.00			$ 25.00				$ 25.00		$ 25.00		$ 25.00
Preferred stock, redemption price (in dollars per share)	$ 25.00							$ 25.00		$ 25.00
Preferred stock (as a percent)										8.50%		8.50%
Minimum number of quarters for which failure to pay dividends on the preferred stock will give rise to voting rights										6
Number of additional directors that can be elected by preferred stock holders										2
Minimum percentage of preferred shareholders required for approval										66.67%
Preferred stock, cash dividends (in dollars per share)		$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125
Dividends on Common Stock
Common stock, cash dividends declared (in dollars per share)	$ 0.210	$ 0.230	$ 0.240	$ 0.270	$ 0.250	$ 0.250	$ 0.235	$ 0.21	$ 0.25	$ 0.68	$ 0.74
Common stock, special dividends declared (in dollars per share)				$ 0.02
Common stock, accrued dividends and DERs payable	$ 76,100,000							$ 76,100,000		$ 76,100,000
Stockholders' Equity
Aggregate number of shares of common stock registered for sale under the plan	895,000,000			895,000,000				895,000,000		895,000,000		895,000,000	10,000,000	10,000,000	10,000,000
Shares of common stock authorized and available for issuance													9,400,000	9,400,000	9,400,000
Shares issued													75,454	596,079	15,146,939		30,144,815
Net proceeds													603,519	4,400,000	132,700,000		194,908,570
Fees and commissions paid to Cantor																	4,189,247
Maximum number of shares that may be offered and sold through Cantor																40,000,000
Accumulated Other Comprehensive Income/(Loss)
Other-than-temporary impairments recognized in accumulated other comprehensive(loss)/income	$ 5,000,000			$ 65,400,000				$ 5,000,000		$ 5,000,000		$ 65,400,000
Number of shares authorized to be repurchased under the Repurchase Program	4,000,000							4,000,000		4,000,000