Derivative Instruments (Details 3)		3 Months Ended								12 Months Ended
		Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) derivative	Dec. 31, 2014 USD ($) derivative	Dec. 31, 2013 USD ($)
Derivatives
Assets pledged		$ 11,410,397,000				$ 10,058,769,000				$ 11,410,397,000	$ 10,058,769,000
Number of derivatives terminated | derivative										0	0
Interest expense attributable to Swaps		46,456,000	$ 43,703,000	$ 42,849,000	$ 43,940,000	38,960,000	$ 39,358,000	$ 40,569,000	$ 40,921,000	$ 176,948,000	$ 159,808,000	$ 164,013,000
(Losses)/gains on TBA short positions										0	0	7,517,000
AOCI from derivative hedging instruments
Balance at beginning of period					$ (59,062,000)				$ (15,217,000)	(59,062,000)	(15,217,000)	(62,831,000)
Unrealized (loss)/gain on Swaps, net										(10,337,000)	(44,292,000)	47,614,000
Reclassification of unrealized loss on de-designated derivative hedging instruments										0	447,000	0
Balance at end of period		(69,399,000)				(59,062,000)				(69,399,000)	(59,062,000)	(15,217,000)
TBA | Non-Hedging
Derivatives
Aggregate notional amount of derivatives		0				0				0	0	$ 350,000,000
Debt instrument, term												15 years
Stated interest rate (as a percent)												2.50%
(Losses)/gains on TBA short positions												$ 7,500,000
Swap contracts
Derivatives
Aggregate fair value of assets needed to immediately settle		72,000,000								72,000,000
Associated accrued interest payable		1,500,000								1,500,000
Assets pledged		109,142,000				123,733,000				109,142,000	123,733,000
Swap contracts | Agency MBS, at fair value
Derivatives
Assets pledged		38,569,000				57,247,000				38,569,000	57,247,000
Swap contracts | Restricted cash
Derivatives
Assets pledged		70,573,000				66,486,000				$ 70,573,000	66,486,000
Swaps
Derivatives
Average maturity term of swaps										45 months
Maximum maturity term of swaps										92 months
Aggregate notional amount of derivatives	[1]	$ 3,050,000,000				$ 3,760,170,000				$ 3,050,000,000	$ 3,760,170,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	1.82%				1.85%				1.82%	1.85%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.34%				0.16%				0.34%	0.16%
Interest expense attributable to Swaps										$ 53,759,000	$ 69,842,000	$ 59,031,000
Weighted average Swap rate paid										1.86%	1.93%	2.08%
Weighted average Swap rate received										0.19%	0.16%	0.19%
Swaps | Hedging
AOCI from derivative hedging instruments
Balance at end of period		$ (69,400,000)								$ (69,400,000)
Swaps | Within 30 days
Derivatives
Aggregate notional amount of derivatives	[1]	$ 50,000,000				$ 22,290,000				$ 50,000,000	$ 22,290,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	2.13%				3.63%				2.13%	3.63%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.42%				0.23%				0.42%	0.23%
Swaps | Over 30 days to 3 months
Derivatives
Aggregate notional amount of derivatives	[1]	$ 0				$ 387,880,000				$ 0	$ 387,880,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	0.00%				1.80%				0.00%	1.80%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.00%				0.16%				0.00%	0.16%
Swaps | Over 3 months to 6 months
Derivatives
Aggregate notional amount of derivatives	[1]	$ 0				$ 300,000,000				$ 0	$ 300,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	0.00%				2.06%				0.00%	2.06%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.00%				0.17%				0.00%	0.17%
Swaps | Over 6 months to 12 months
Derivatives
Aggregate notional amount of derivatives	[1]	$ 100,000,000				$ 0				$ 100,000,000	$ 0
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	0.48%				0.00%				0.48%	0.00%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.32%				0.00%				0.32%	0.00%
Swaps | Over 12 months to 24 months
Derivatives
Aggregate notional amount of derivatives	[1]	$ 350,000,000				$ 150,000,000				$ 350,000,000	$ 150,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	0.58%				1.03%				0.58%	1.03%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.27%				0.16%				0.27%	0.16%
Swaps | Over 24 months to 36 months
Derivatives
Aggregate notional amount of derivatives	[1]	$ 550,000,000				$ 350,000,000				$ 550,000,000	$ 350,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	1.49%				0.58%				1.49%	0.58%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.32%				0.16%				0.32%	0.16%
Swaps | Over 36 months to 48 months
Derivatives
Aggregate notional amount of derivatives	[1]	$ 200,000,000				$ 550,000,000				$ 200,000,000	$ 550,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	1.71%				1.49%				1.71%	1.49%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.42%				0.16%				0.42%	0.16%
Swaps | Over 48 months to 60 months
Derivatives
Aggregate notional amount of derivatives	[1]	$ 1,500,000,000				$ 200,000,000				$ 1,500,000,000	$ 200,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	2.22%				1.71%				2.22%	1.71%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.36%				0.17%				0.36%	0.17%
Swaps | Over 60 months to 72 months
Derivatives
Aggregate notional amount of derivatives	[1]	$ 200,000,000				$ 1,500,000,000				$ 200,000,000	$ 1,500,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	2.20%				2.22%				2.20%	2.22%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.30%				0.16%				0.30%	0.16%
Swaps | Over 72 months to 84 months
Derivatives
Aggregate notional amount of derivatives	[1]	$ 0				$ 200,000,000				$ 0	$ 200,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1]	0.00%				2.20%				0.00%	2.20%
Weighted Average Variable Interest Rate (as a percent)	[1],[2]	0.00%				0.17%				0.00%	0.17%
Swaps | Over 84 months
Derivatives
Aggregate notional amount of derivatives	[1],[3]	$ 100,000,000				$ 100,000,000				$ 100,000,000	$ 100,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	[1],[3]	2.75%				2.75%				2.75%	2.75%
Weighted Average Variable Interest Rate (as a percent)	[1],[2],[3]	0.40%				0.16%				0.40%	0.16%
Swaps | New Swaps
Derivatives
Notional amount of derivative entered										$ 0
Number of new derivatives | derivative										0
Weighted Average Fixed-Pay Interest Rate (as a percent)		0.00%								0.00%
Swaps | Swaps amortized expired
Derivatives
Aggregate notional amount of derivatives expired										$ 710,170,000
Swaps | Minimum | London Interbank Offered Rate (LIBOR)
Derivatives
Derivative, variable interest rate, term										1 month
Swaps | Maximum | London Interbank Offered Rate (LIBOR)
Derivatives
Derivative, variable interest rate, term										3 months
YTD | Swaps | Swaps amortized expired
Derivatives
Weighted Average Fixed-Pay Interest Rate (as a percent)		1.96%								1.96%

[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.