Derivatives (Details 3) (USD $)	3 Months Ended
	Mar. 31, 2013 derivative	Mar. 31, 2012 derivative	Dec. 31, 2012
Derivatives
Assets Pledged	$ 10,970,161,000		$ 10,624,276,000
Number of derivatives terminated	0	0
Interest Expense	40,158,000	40,127,000
Impact of Hedging Instruments on AOCI
Balance at beginning of period	(62,831,000)	(114,194,000)
Unrealized gain on Swaps, net	12,316,000	12,091,000
Balance at end of period	(50,515,000)	(102,103,000)
Derivative contracts
Derivatives
Assets Pledged	62,136,000		73,931,000
Swaps
Derivatives
Aggregate fair value of assets needed to immediately settle	53,100,000
Associated accrued interest payable	2,200,000
Aggregate notional amount of derivatives	2,513,550,000		2,519,564,000
Interest rate, description	one-month or three-month London Interbank Offered Rate (LIBOR)
Net unrealized losses	50,500,000
Average maturity term of swaps	18 months
Maximum maturity term of swaps	47 months
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.24%		2.31%
Weighted Average Variable Interest Rate (as a percent)	0.21%		0.22%
Interest Expense	12,969,000	20,823,000
Weighted average Swap rate paid (as a percent)	2.27%	2.78%
Weighted average Swap rate received (as a percent)	0.21%	0.31%
Number of new derivatives	1
Notional amount of derivative entered	50,000,000
Fixed-pay rate (as a percent)	0.67%
Initial maturity of derivative entered	4 years
Aggregate notional amount of derivatives expired	56,000,000
Swaps | Within 30 days.
Derivatives
Aggregate notional amount of derivatives	287,832,000		25,828,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.85%		3.88%
Weighted Average Variable Interest Rate (as a percent)	0.21%		0.28%
Swaps | Over 30 days to 3 months
Derivatives
Aggregate notional amount of derivatives	239,443,000		30,185,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.36%		3.96%
Weighted Average Variable Interest Rate (as a percent)	0.21%		0.26%
Swaps | Over 3 months to 6 months
Derivatives
Aggregate notional amount of derivatives	357,012,000		527,275,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	4.18%		1.63%
Weighted Average Variable Interest Rate (as a percent)	0.21%		0.21%
Swaps | Over 6 months to 12 months
Derivatives
Aggregate notional amount of derivatives	75,902,000		391,063,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	4.00%		4.17%
Weighted Average Variable Interest Rate (as a percent)	0.25%		0.22%
Swaps | Over 12 months to 24 months
Derivatives
Aggregate notional amount of derivatives	1,053,361,000		685,042,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.06%		2.28%
Weighted Average Variable Interest Rate (as a percent)	0.21%		0.22%
Swaps | Over 24 months to 36 months
Derivatives
Aggregate notional amount of derivatives	350,000,000		710,171,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.07%		1.97%
Weighted Average Variable Interest Rate (as a percent)	0.20%		0.21%
Swaps | Over 36 months to 48 months
Derivatives
Aggregate notional amount of derivatives	150,000,000		150,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	0.54%		1.03%
Weighted Average Variable Interest Rate (as a percent)	0.20%		0.21%
Agency MBS, at fair value | Derivative contracts
Derivatives
Assets Pledged	57,119,000		68,915,000
Restricted cash | Derivative contracts
Derivatives
Assets Pledged	$ 5,017,000		$ 5,016,000