Derivative Instruments (Details 3) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2014 derivative		Jun. 30, 2013 derivative	Jun. 30, 2014		Jun. 30, 2013 derivative	Dec. 31, 2013
Derivative Instruments
Assets pledged	$ 10,349,034,000			$ 10,349,034,000			$ 10,227,092,000
Restricted cash	54,430,000			54,430,000			37,520,000
Number of derivatives terminated	0		0			0
Interest expense attributable to Swaps	40,569,000		38,478,000	81,490,000		78,636,000
Impact of Hedging Instruments on AOCI
Balance at beginning of period	(27,037,000)		(50,515,000)	(15,217,000)		(62,831,000)
Unrealized (loss)/gain on Swaps, net	(27,634,000)		18,548,000	(39,901,000)		30,864,000
Reclassification of unrealized loss on de-designated Swaps	0		0	447,000		0
Balance at end of period	(54,671,000)		(31,967,000)	(54,671,000)		(31,967,000)
Interest Rate Contract
Derivative Instruments
Aggregate fair value of assets needed to immediately settle	59,700,000			59,700,000
Associated accrued interest payable	2,300,000			2,300,000
Assets pledged	127,691,000			127,691,000			111,379,000
Swaps
Derivative Instruments
Aggregate notional amount of derivatives	3,927,214,000	[1]		3,927,214,000	[1]		4,045,212,000	[1]
Interest rate, description				one-month or three-month London Interbank Offered Rate (LIBOR)
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.91%	[1]		1.91%	[1]		1.91%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.15%	[1],[2]		0.15%	[1],[2]		0.17%	[1],[2]
Interest expense attributable to Swaps	18,075,000		12,027,000	35,638,000		24,996,000
Weighted average Swap rate paid (as a percent)	1.92%		2.12%	1.92%		2.22%
Weighted average Swap rate received (as a percent)	0.15%		0.20%	0.16%		0.20%
Swaps | Minimum | LIBOR
Derivative Instruments
Derivative, variable interest rate, term				1 month
Swaps | Maximum | LIBOR
Derivative Instruments
Derivative, variable interest rate, term				3 months
Swaps | Within 30 days.
Derivative Instruments
Aggregate notional amount of derivatives	6,978,000	[1]		6,978,000	[1]		17,635,000	[1]
Weighted Average Fixed-Pay Interest Rate (as a percent)	4.07%	[1]		4.07%	[1]		3.90%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.20%	[1],[2]		0.20%	[1],[2]		0.21%	[1],[2]
Swaps | Over 30 days to 3 months
Derivative Instruments
Aggregate notional amount of derivatives	39,344,000	[1]		39,344,000	[1]		24,216,000	[1]
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.63%	[1]		2.63%	[1]		3.93%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.16%	[1],[2]		0.16%	[1],[2]		0.21%	[1],[2]
Swaps | Over 3 months to 6 months
Derivative Instruments
Aggregate notional amount of derivatives	120,722,000	[1]		120,722,000	[1]		476,147,000	[1]
Weighted Average Fixed-Pay Interest Rate (as a percent)	3.36%	[1]		3.36%	[1]		1.80%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.17%	[1],[2]		0.17%	[1],[2]		0.17%	[1],[2]
Swaps | Over 6 months to 12 months
Derivative Instruments
Aggregate notional amount of derivatives	710,170,000	[1]		710,170,000	[1]		167,043,000	[1]
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.96%	[1]		1.96%	[1]		3.22%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.16%	[1],[2]		0.16%	[1],[2]		0.18%	[1],[2]
Swaps | Over 12 months to 24 months
Derivative Instruments
Aggregate notional amount of derivatives	50,000,000	[1]		50,000,000	[1]		710,171,000	[1]
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.13%	[1]		2.13%	[1]		1.97%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.15%	[1],[2]		0.15%	[1],[2]		0.17%	[1],[2]
Swaps | Over 24 months to 36 months
Derivative Instruments
Aggregate notional amount of derivatives	450,000,000	[1]		450,000,000	[1]		150,000,000	[1]
Weighted Average Fixed-Pay Interest Rate (as a percent)	0.56%	[1]		0.56%	[1]		1.03%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.15%	[1],[2]		0.15%	[1],[2]		0.17%	[1],[2]
Swaps | Over 36 months to 48 months
Derivative Instruments
Aggregate notional amount of derivatives	50,000,000	[1]		50,000,000	[1]		350,000,000	[1]
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.45%	[1]		1.45%	[1]		0.58%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.15%	[1],[2]		0.15%	[1],[2]		0.17%	[1],[2]
Swaps | Over 48 Months to 60 Months
Derivative Instruments
Aggregate notional amount of derivatives	700,000,000	[1]		700,000,000	[1]		550,000,000	[1]
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.56%	[1]		1.56%	[1]		1.49%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.15%	[1],[2]		0.15%	[1],[2]		0.17%	[1],[2]
Swaps | Over 60 Months to 72 Months
Derivative Instruments
Aggregate notional amount of derivatives	200,000,000	[1]		200,000,000	[1]		0	[1]
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.05%	[1]		2.05%	[1]		0.00%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.15%	[1],[2]		0.15%	[1],[2]		0.00%	[1],[2]
Swaps | Over 72 Months to 84 Months
Derivative Instruments
Aggregate notional amount of derivatives	1,500,000,000	[1]		1,500,000,000	[1]		1,500,000,000	[1]
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.24%	[1]		2.24%	[1]		2.22%	[1]
Weighted Average Variable Interest Rate (as a percent)	0.15%	[1],[2]		0.15%	[1],[2]		0.17%	[1],[2]
Swaps | Over 84 Months
Derivative Instruments
Aggregate notional amount of derivatives	100,000,000	[1],[3]		100,000,000	[1],[3]		100,000,000	[1],[3]
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.75%	[1],[3]		2.75%	[1],[3]		2.75%	[1],[3]
Weighted Average Variable Interest Rate (as a percent)	0.16%	[1],[2],[3]		0.16%	[1],[2],[3]		0.17%	[1],[2],[3]
Agency MBS, at fair value | Interest Rate Contract
Derivative Instruments
Assets pledged	75,061,000			75,061,000			73,859,000
Restricted Cash | Interest Rate Contract
Derivative Instruments
Restricted cash	52,630,000			52,630,000			37,520,000
New Swaps | Swaps
Derivative Instruments
Number of new swaps	2			4
Notional amount of new swaps	200,000,000			400,000,000
New Swaps | Swaps | Minimum
Derivative Instruments
Initial maturity date	5 years			5 years
New Swaps | Swaps | Maximum
Derivative Instruments
Initial maturity date	7 years			7 years
Swaps amortized expired | Swaps
Derivative Instruments
Aggregate notional amount of derivatives expired	476,147,000			517,998,000
Hedging | Swaps
Derivative Instruments
Aggregate notional amount of derivatives	3,927,000,000	[1]		3,927,000,000	[1]
Average maturity term of swaps				51 months
Maximum maturity term of swaps				110 months
Impact of Hedging Instruments on AOCI
Balance at end of period	$ 54,700,000			$ 54,700,000
QTD | New Swaps | Swaps
Derivative Instruments
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.96%			1.96%
QTD | Swaps amortized expired | Swaps
Derivative Instruments
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.80%			1.80%
YTD | New Swaps | Swaps
Derivative Instruments
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.95%			1.95%
YTD | Swaps amortized expired | Swaps
Derivative Instruments
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.97%			1.97%

[1] Each maturity category reflects contractual amortization and/or maturity of notional amounts.

[2] Reflects the benchmark variable rate due from the counterparty at the date presented, which rate adjusts monthly or quarterly based on one-month or three-month LIBOR, respectively.

[3] Reflects one Swap with a maturity date of July 2023.