Derivatives (Details 3) (USD $)	3 Months Ended		6 Months Ended																		6 Months Ended		3 Months Ended		6 Months Ended		1 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Within 30 days.	Dec. 31, 2011 Within 30 days.	Jun. 30, 2012 Over 30 days to 3 months	Dec. 31, 2011 Over 30 days to 3 months	Jun. 30, 2012 Over 3 months to 6 months	Dec. 31, 2011 Over 3 months to 6 months	Jun. 30, 2012 Over 6 months to 12 months	Dec. 31, 2011 Over 6 months to 12 months	Jun. 30, 2012 Over 12 months to 24 months	Dec. 31, 2011 Over 12 months to 24 months	Jun. 30, 2012 Over 24 months to 36 months	Dec. 31, 2011 Over 24 months to 36 months	Jun. 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	Jun. 30, 2012 Derivative contracts	Dec. 31, 2011 Derivative contracts	Jun. 30, 2012 Swaps	Jun. 30, 2011 Swaps	Jun. 30, 2012 Swaps	Jun. 30, 2011 Swaps	Jun. 30, 2011 Swaptions	Jun. 30, 2011 Swaptions	Jun. 30, 2012 Swaptions M Y	Jun. 30, 2011 Swaptions	Jun. 30, 2012 Agency MBS, at fair value Derivative contracts	Dec. 31, 2011 Agency MBS, at fair value Derivative contracts	Jun. 30, 2012 Restricted cash Derivative contracts	Dec. 31, 2011 Restricted cash Derivative contracts
Derivatives
Aggregate fair value of assets needed to immediately settle																					$ 93,300,000
Assets Pledged	9,920,016,000		9,920,016,000		9,056,087,000																104,256,000	133,189,000									95,247,000	117,687,000	9,009,000	15,502,000
Aggregate notional amount of derivatives	2,948,459,000		2,948,459,000		3,377,866,000	32,593,000	34,056,000	155,150,000	120,001,000	341,152,000	275,351,000	583,288,000	528,894,000	909,062,000	974,352,000	877,214,000	685,042,000	50,000,000	710,170,000	50,000,000			2,948,000,000		2,948,000,000				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																							89,800,000		89,800,000
Average maturity term of swaps																							18 months		18 months
Maximum maturity term of swaps																							43 months		43 months
Aggregate notional amount of derivatives expired																							275,400,000		429,400,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.75%		2.75%		2.80%	4.06%	4.05%	4.48%	4.43%	4.43%	2.54%	1.85%	4.42%	2.92%	2.78%	2.20%	2.28%	2.13%	1.96%	2.13%
Weighted Average Variable Interest Rate (as a percent)	0.28%		0.28%		0.32%	0.35%	0.37%	0.30%	0.38%	0.38%	0.33%	0.26%	0.39%	0.26%	0.30%	0.26%	0.31%	0.25%	0.29%	0.29%
Interest Expense	42,688,000	37,195,000	82,815,000	71,848,000																			19,314,000	24,725,000	40,137,000	48,759,000
Weighted average Swap rate paid (as a percent)																							2.70%	3.26%	2.74%	3.45%
Weighted average Swap rate received (as a percent)																							0.28%	0.23%	0.29%	0.25%
Premium paid on purchase of derivative																											915,000
Term of derivative (in years)																													4
Fixed strike rate (as a percent)																													1.90%
Period of fixed rate borrowings (in months)																													1
Fair value					26,000																								1,200,000
Impact of Hedging Instruments on Accumulated Other Comprehensive Income/(Loss)
Balance at beginning of period	(102,103,000)	(113,471,000)	(114,194,000)	(139,142,000)
Unrealized gain (loss) on derivative	12,280,000	(10,933,000)	24,371,000	14,738,000																			12,280,000	(11,160,000)	24,371,000	14,511,000		227,000		227,000
Income (loss) due to change in time value component of derivative																												12,000		12,000
Balance at end of period	$ (89,823,000)	$ (124,404,000)	$ (89,823,000)	$ (124,404,000)