Derivatives (Details 3) (USD $)	3 Months Ended								12 Months Ended					12 Months Ended																		1 Months Ended		12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Derivative contracts	Dec. 31, 2011 Derivative contracts	Dec. 31, 2012 Swaps derivative	Dec. 31, 2011 Swaps	Dec. 31, 2010 Swaps	Dec. 31, 2012 Swaps Within 30 days.	Dec. 31, 2011 Swaps Within 30 days.	Dec. 31, 2012 Swaps Over 30 days to 3 months	Dec. 31, 2011 Swaps Over 30 days to 3 months	Dec. 31, 2012 Swaps Over 3 months to 6 months	Dec. 31, 2011 Swaps Over 3 months to 6 months	Dec. 31, 2012 Swaps Over 6 months to 12 months	Dec. 31, 2011 Swaps Over 6 months to 12 months	Dec. 31, 2012 Swaps Over 12 months to 24 months	Dec. 31, 2011 Swaps Over 12 months to 24 months	Dec. 31, 2012 Swaps Over 24 months to 36 months	Dec. 31, 2011 Swaps Over 24 months to 36 months	Dec. 31, 2012 Swaps Over 36 months to 48 months	Dec. 31, 2011 Swaps Over 36 months to 48 months	Dec. 31, 2011 Swaps Over 48 months to 60 months	Jan. 31, 2012 Swaptions	Jun. 30, 2011 Swaptions	Dec. 31, 2011 Swaptions	Dec. 31, 2012 Agency MBS, at fair value Derivative contracts	Dec. 31, 2011 Agency MBS, at fair value Derivative contracts	Dec. 31, 2012 Restricted cash Derivative contracts	Dec. 31, 2011 Restricted cash Derivative contracts
Derivatives
Aggregate fair value of assets needed to immediately settle												$ 65,500,000
Associated accrued interest payable												2,400,000
Assets Pledged	10,624,276,000				9,056,087,000				10,624,276,000	9,056,087,000		73,931,000	133,189,000																						68,915,000	117,687,000	5,016,000	15,502,000
Aggregate notional amount of derivatives														2,519,564,000	3,377,866,000		25,828,000	34,056,000	30,185,000	120,001,000	527,275,000	275,351,000	391,063,000	528,894,000	685,042,000	974,352,000	710,171,000	685,042,000	150,000,000	710,170,000	50,000,000
Interest rate, description														one-month or three-month London Interbank Offered Rate (LIBOR)
Interest rate basis for swap the company may purchase																																	one-month London Interbank Offered Rate ("LIBOR")
Net unrealized losses														62,800,000
Average maturity term of swaps														17 months
Maximum maturity term of swaps														48 months
Weighted Average Fixed-Pay Interest Rate (as a percent)														2.31%	2.80%		3.88%	4.05%	3.96%	4.43%	1.63%	2.54%	4.17%	4.42%	2.28%	2.78%	1.97%	2.28%	1.03%	1.96%	2.13%
Weighted Average Variable Interest Rate (as a percent)														0.22%	0.32%		0.28%	0.37%	0.26%	0.38%	0.21%	0.33%	0.22%	0.39%	0.22%	0.30%	0.21%	0.31%	0.21%	0.29%	0.29%
Interest Expense	43,054,000	45,801,000	42,688,000	40,127,000	38,811,000	38,752,000	37,195,000	34,653,000	171,670,000	149,411,000	145,125,000			73,266,000	95,677,000	111,791,000
Weighted average Swap rate paid (as a percent)														2.68%	3.13%	3.97%
Weighted average Swap rate received (as a percent)														0.27%	0.25%	0.30%
Number of new derivatives														1
Notional amount of derivative entered														100,000,000
Fixed-pay rate (as a percent)														0.48%
Initial maturity of derivative entered														4 years
Premium paid on purchase of derivative																																	915,000
Aggregate notional amount of derivatives expired														958,300,000																		100,000,000
Term of swap the company may purchase																																	4 years
Fixed strike rate (as a percent)																																	1.90%
Period of fixed rate borrowings hedged against																																	1 month
Expense recognized in other income																																		915,000
Impact of Hedging Instruments on Accumulated Other Comprehensive Income/(Loss)
Balance at beginning of period				(114,194,000)				(139,142,000)	(114,194,000)	(139,142,000)	(152,463,000)
Unrealized gain on Swaps, net									51,363,000	24,948,000	13,321,000
Balance at end of period	$ (62,831,000)				$ (114,194,000)				$ (62,831,000)	$ (114,194,000)	$ (139,142,000)