Stockholders' Equity (Details) (USD $)	1 Months Ended																		12 Months Ended						12 Months Ended			100 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended	88 Months Ended
	Dec. 31, 2011 Quarter	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Oct. 31, 2010	Sep. 30, 2010	Jul. 31, 2010	Jun. 30, 2010	Apr. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Oct. 31, 2009	Sep. 30, 2009	Jul. 31, 2009	Jun. 30, 2009	Apr. 30, 2009	Mar. 31, 2009	Dec. 31, 2011 Director Quarter	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2011 Swaps	Dec. 31, 2010 Swaps	Dec. 31, 2011 DRSPP	Dec. 31, 2010 DRSPP	Dec. 31, 2009 DRSPP	Dec. 31, 2011 DRSPP	Dec. 31, 2011 Public Offering of Common Stock	Dec. 31, 2009 Public Offering of Common Stock	Dec. 31, 2008 Controlled Equity Offering Program	Dec. 31, 2009 Controlled Equity Offering Program	Dec. 31, 2011 Controlled Equity Offering Program
Dividends on Preferred Stock
Preferred stock, shares issued	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
Preferred stock, par value (in dollars per share)	$ 0.01				$ 0.01														$ 0.01	$ 0.01
Preferred stock, liquidation preference (in dollars per share)	$ 25				$ 25							$ 25							$ 25	$ 25	$ 25
Preferred stock, redemption price (in dollars per share)	$ 25																		$ 25
Preferred stock, dividend rate (as a percent)																			8.50%
Minimum number of quarters for which failure to pay dividends on the preferred stock will give rise to voting rights	6																		6
Number of additional directors that can be elected by preferred stock holders																			2
Minimum percentage of preferred shareholders required for approval (as a percent)																			66.67%
Preferred stock, cash dividends (in dollars per share)	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125		$ 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)	$ 0.270	$ 0.250	$ 0.250	$ 0.235	$ 0.235	$ 0.225		$ 0.190		$ 0.240		$ 0.270	$ 0.250		$ 0.250		$ 0.220
Common stock, special dividends declared (in dollars per share)	$ 0.02
Stockholders' Equity
Aggregate number of shares of common stock registered for sale under the plan	895,000,000				370,000,000														895,000,000	370,000,000					10,000,000			10,000,000
Shares of common stock authorized and available for issuance (in shares)																									10,000,000			10,000,000
Shares issued (in shares)																									404,516,000	80,138,000	59,090,000	14,550,860	74,750,000	57,500,000		2,810,000	30,144,815
Gross Proceeds Per Share (in dollars per share)																													$ 8.10	$ 7.05
Gross Proceeds																			$ 607,803,000	$ 609,000	$ 403,368,000								$ 605,475,000	$ 405,375,000
Expenses incurred in connection with equity offering																													438,000
Underwriting discounts and related expenses																														18,600,000
Net proceeds																									2,801,318	589,979	394,854	128,300,000				16,355,764	194,908,570
Fees and commissions paid to Cantor																																	4,189,247
Maximum number of shares that may be offered and sold through Cantor (in shares)																															40,000,000
Available-for-sale MBS:
Unrealized gains	345,200,000				421,969,000														345,200,000	421,969,000
Unrealized losses	(289,709,000)				(28,147,000)														(289,709,000)	(28,147,000)
Net unrealized gain (loss) on Available-for-sale MBS	55,491,000				393,822,000														55,491,000	393,822,000
Derivative hedging instruments:
Unrealized gains																							26,000
Unrealized losses																							(114,220,000)	(139,142,000)
Net unrealized gain (loss) on Hedging instruments	(114,194,000)				(139,142,000)							(152,463,000)							(114,194,000)	(139,142,000)	(152,463,000)	(237,291,000)
Accumulated other comprehensive (loss)/income	(58,703,000)				254,680,000														(58,703,000)	254,680,000
Other-than-temporary impairments recognized in accumulated other comprehensive(loss)/income	$ 65,400,000				$ 12,000,000														$ 65,400,000	$ 12,000,000
Number of shares authorized to be repurchased under the Repurchase Program (in shares)	4,000,000																		4,000,000