Derivatives (Details 3) (USD $)	3 Months Ended		9 Months Ended									3 Months Ended		9 Months Ended			1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 Agency MBS, at fair value Derivative contracts	Dec. 31, 2010 Agency MBS, at fair value Derivative contracts	Sep. 30, 2011 Restricted cash Derivative contracts	Dec. 31, 2010 Restricted cash Derivative contracts	Sep. 30, 2011 Derivative contracts	Dec. 31, 2010 Derivative contracts	Sep. 30, 2011 Swaps	Sep. 30, 2010 Swaps	Sep. 30, 2011 Swaps	Sep. 30, 2010 Swaps	Dec. 31, 2010 Swaps	Jun. 30, 2011 Swaptions	Sep. 30, 2011 Swaptions M Y	Sep. 30, 2011 Swaptions M Y	Sep. 30, 2011 Within 30 days.	Dec. 31, 2010 Within 30 days.	Sep. 30, 2011 Over 30 days to 3 months	Dec. 31, 2010 Over 30 days to 3 months	Sep. 30, 2011 Over 3 months to 6 months	Dec. 31, 2010 Over 3 months to 6 months	Sep. 30, 2011 Over 6 months to 12 months	Dec. 31, 2010 Over 6 months to 12 months	Sep. 30, 2011 Over 12 months to 24 months	Dec. 31, 2010 Over 12 months to 24 months	Sep. 30, 2011 Over 24 months to 36 months	Dec. 31, 2010 Over 24 months to 36 months	Sep. 30, 2011 Over 36 months to 48 months	Dec. 31, 2010 Over 36 months to 48 months	Sep. 30, 2011 Over 48 months to 60 months	Dec. 31, 2010 Over 48 months to 60 months
Derivatives
Aggregate fair value of assets needed to immediately settle										$ 134,700,000
Assets Pledged	9,417,754,000		9,417,754,000		6,891,431,000	146,380,000	153,534,000	22,498,000	35,083,000	168,878,000	188,617,000
Aggregate notional amount of derivatives	3,503,964,000		3,503,964,000		2,805,461,000							3,504,000,000		3,504,000,000				100,000,000	100,000,000	36,696,000	55,267,000	89,402,000	160,589,000	154,057,000	169,258,000	463,094,000	257,482,000	1,281,452,000	833,302,000	598,371,000	849,351,000	830,892,000	360,042,000	50,000,000	120,170,000
Interest rate, description														one-month or three-month London Interbank Offered Rate ("LIBOR")			one month LIBOR
Gross unrealized losses												134,700,000		134,700,000		139,142,000
Gross unrealized gains												53,000		53,000
Average maturity term of swaps												24 months		24 months
Maximum maturity term of swaps												53 months		53 months
Aggregate notional amount of derivatives entered												20,000,000		1,215,000,000
Aggregate notional amount of derivatives expired												131,400,000		516,500,000
Weighted Average Fixed-Pay Interest Rate	2.85%		2.85%		3.74%															4.06%	3.90%	4.16%	4.35%	4.35%	4.02%	3.30%	4.09%	3.18%	4.40%	2.16%	3.10%	2.17%	3.32%	2.13%	2.87%
Weighted Average Variable Interest Rate (as a percent)	0.25%		0.25%		0.27%															0.25%	0.28%	0.29%	0.27%	0.26%	0.28%	0.25%	0.28%	0.25%	0.27%	0.24%	0.26%	0.24%	0.27%	0.24%	0.27%
Interest Expense	38,752,000	35,464,000	110,600,000	109,656,000								24,322,000	27,758,000	73,091,000	85,474,000
Weighted average Swap rate paid (as a percent)												2.88%	3.84%	3.24%	4.02%
Weighted average Swap rate received (as a percent)												0.21%	0.36%	0.24%	0.30%
Premium paid on purchase of derivative																	915,000
Term of derivative (in years)																		4	4
Fixed strike rate (as a percent)																		1.90%	1.90%
Period of fixed rate borrowings (in months)																		1	1
Derivative assets, fair value	81,000		81,000															28,000	28,000
Impact of Hedging Instruments on Accumulated Other Comprehensive Income
Balance at beginning of period	(124,404,000)	(167,679,000)	(139,142,000)	(152,463,000)
Unrealized (loss)/gain on derivative	(10,255,000)	(7,624,000)	4,483,000	(22,840,000)								(10,028,000)	(7,624,000)	4,483,000	(22,840,000)			(227,000)	227,000
Balance at end of period	(134,659,000)	(175,303,000)	(134,659,000)	(175,303,000)
Income (loss) due to change in time value component of derivative																		227,000
Expense recognized in other income																		$ 899,000	$ 887,000