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, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Within 30 days.	Dec. 31, 2011 Within 30 days.	Sep. 30, 2012 Over 30 days to 3 months	Dec. 31, 2011 Over 30 days to 3 months	Sep. 30, 2012 Over 3 months to 6 months	Dec. 31, 2011 Over 3 months to 6 months	Sep. 30, 2012 Over 6 months to 12 months	Dec. 31, 2011 Over 6 months to 12 months	Sep. 30, 2012 Over 12 months to 24 months	Dec. 31, 2011 Over 12 months to 24 months	Sep. 30, 2012 Over 24 months to 36 months	Dec. 31, 2011 Over 24 months to 36 months	Sep. 30, 2012 Over 36 months to 48 months	Dec. 31, 2011 Over 36 months to 48 months	Dec. 31, 2011 Over 48 months to 60 months	Sep. 30, 2012 Derivative contracts	Dec. 31, 2011 Derivative contracts	Sep. 30, 2012 Swaps	Sep. 30, 2011 Swaps	Sep. 30, 2012 Swaps	Sep. 30, 2011 Swaps	Jun. 30, 2011 Swaptions	Sep. 30, 2011 Swaptions	Sep. 30, 2012 Swaptions	Sep. 30, 2011 Swaptions	Sep. 30, 2012 Agency MBS, at fair value Derivative contracts	Dec. 31, 2011 Agency MBS, at fair value Derivative contracts	Sep. 30, 2012 Restricted cash Derivative contracts	Dec. 31, 2011 Restricted cash Derivative contracts
Derivatives
Aggregate fair value of assets needed to immediately settle																					$ 81,500,000
Associated accrued interest payable																					3,300,000
Assets Pledged	10,635,546,000		10,635,546,000		9,056,087,000																91,163,000	133,189,000									84,150,000	117,687,000	7,013,000	15,502,000
Aggregate notional amount of derivatives	2,760,715,000		2,760,715,000		3,377,866,000	129,533,000	34,056,000	211,618,000	120,001,000	56,013,000	275,351,000	884,287,000	528,894,000	598,372,000	974,352,000	830,892,000	685,042,000	50,000,000	710,170,000	50,000,000			2,761,000,000		2,761,000,000				100,000,000
Interest rate, description																									one-month or three-month London Interbank Offered Rate (LIBOR)				one-month London Interbank Offered Rate ("LIBOR")
Gross unrealized losses																							78,200,000		78,200,000
Average maturity term of swaps																									16 months
Maximum maturity term of swaps																									40 months
Aggregate notional amount of derivatives expired																							187,700,000		617,200,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.64%		2.64%		2.80%	4.56%	4.05%	4.35%	4.43%	3.92%	2.54%	2.66%	4.42%	2.16%	2.78%	2.17%	2.28%	2.13%	1.96%	2.13%
Weighted Average Variable Interest Rate (as a percent)	0.25%		0.25%		0.32%	0.24%	0.37%	0.41%	0.38%	0.34%	0.33%	0.23%	0.39%	0.24%	0.30%	0.23%	0.31%	0.22%	0.29%	0.29%
Interest Expense	45,801,000	38,752,000	128,616,000	110,600,000																			18,138,000	24,322,000	58,275,000	73,091,000
Weighted average Swap rate paid (as a percent)																							2.72%	2.88%	2.74%	3.24%
Weighted average Swap rate received (as a percent)																							0.27%	0.21%	0.29%	0.24%
Premium paid on purchase of derivative																											915,000
Term of derivative																													4 years
Fixed strike rate (as a percent)																													1.90%
Period of fixed rate borrowings																													1 month
Fair value					26,000																								28,000
Impact of Hedging Instruments on Accumulated Other Comprehensive Income/(Loss)
Balance at beginning of period	(89,823,000)	(124,404,000)	(114,194,000)	(139,142,000)
Unrealized gain (loss) on derivative	11,654,000	(10,255,000)	36,025,000	4,483,000																			11,654,000	(10,028,000)	36,025,000	4,483,000		(227,000)
Income (loss) due to change in time value component of derivative																												899,000		887,000
Balance at end of period	$ (78,169,000)	$ (134,659,000)	$ (78,169,000)	$ (134,659,000)