Derivative Instruments (Details 3) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) derivative	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) derivative	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative Instruments
Assets pledged					$ 11,583,809	$ 10,058,769
Restricted cash					68,057	67,255
Net unrealized gains (losses)	$ (91,429)	$ (27,037)	$ (59,062)	$ (15,217)	(64,571)	(59,062)
Interest expense attributable to Swaps	42,849	40,569	86,789	81,490
AOCI from derivative hedging instruments:
Balance at beginning of period	(91,429)	(27,037)	(59,062)	(15,217)
Unrealized gain/(loss) on Swaps, net	26,858	(27,634)	(5,509)	(39,901)
Reclassification of unrealized loss on de-designated Swaps	0	0	0	447
Balance at end of period	(64,571)	(54,671)	$ (64,571)	(54,671)
Interest Rate Contract
Derivative Instruments
Aggregate fair value of assets needed to immediately settle					67,000
Associated accrued interest payable					1,600
Assets pledged					111,954	123,733
Swaps
Derivative Instruments
Interest rate, description
Aggregate notional amount of derivatives					$ 3,050,000	$ 3,760,170
Weighted Average Fixed-Pay Interest Rate (as a percent)					1.82%	1.85%
Weighted Average Variable Interest Rate (as a percent)					0.19%	0.16%
Interest expense attributable to Swaps	$ 13,174	$ 18,075	$ 28,555	$ 35,638
Weighted average Swap rate paid (as a percent)	1.83%	1.92%	1.84%	1.92%
Weighted average Swap rate received (as a percent)	0.18%	0.15%	0.18%	0.16%
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					$ 0	$ 22,290
Weighted Average Fixed-Pay Interest Rate (as a percent)					0.00%	3.63%
Weighted Average Variable Interest Rate (as a percent)					0.00%	0.23%
Swaps | Over 30 days to 3 months
Derivative Instruments
Aggregate notional amount of derivatives					$ 0	$ 387,880
Weighted Average Fixed-Pay Interest Rate (as a percent)					0.00%	1.80%
Weighted Average Variable Interest Rate (as a percent)					0.00%	0.16%
Swaps | Over 3 months to 6 months
Derivative Instruments
Aggregate notional amount of derivatives					$ 0	$ 300,000
Weighted Average Fixed-Pay Interest Rate (as a percent)					0.00%	2.06%
Weighted Average Variable Interest Rate (as a percent)					0.00%	0.17%
Swaps | Over 6 months to 12 months
Derivative Instruments
Aggregate notional amount of derivatives					$ 50,000	$ 0
Weighted Average Fixed-Pay Interest Rate (as a percent)					2.13%	0.00%
Weighted Average Variable Interest Rate (as a percent)					0.19%	0.00%
Swaps | Over 12 months to 24 months
Derivative Instruments
Aggregate notional amount of derivatives					$ 450,000	$ 150,000
Weighted Average Fixed-Pay Interest Rate (as a percent)					0.56%	1.03%
Weighted Average Variable Interest Rate (as a percent)					0.18%	0.16%
Swaps | Over 24 months to 36 months
Derivative Instruments
Aggregate notional amount of derivatives					$ 50,000	$ 350,000
Weighted Average Fixed-Pay Interest Rate (as a percent)					1.45%	0.58%
Weighted Average Variable Interest Rate (as a percent)					0.19%	0.16%
Swaps | Over 36 months to 48 months
Derivative Instruments
Aggregate notional amount of derivatives					$ 700,000	$ 550,000
Weighted Average Fixed-Pay Interest Rate (as a percent)					1.56%	1.49%
Weighted Average Variable Interest Rate (as a percent)					0.19%	0.16%
Swaps | Over 48 months to 60 months
Derivative Instruments
Aggregate notional amount of derivatives					$ 200,000	$ 200,000
Weighted Average Fixed-Pay Interest Rate (as a percent)					2.05%	1.71%
Weighted Average Variable Interest Rate (as a percent)					0.19%	0.17%
Swaps | Over 60 months to 72 months
Derivative Instruments
Aggregate notional amount of derivatives					$ 1,500,000	$ 1,500,000
Weighted Average Fixed-Pay Interest Rate (as a percent)					2.24%	2.22%
Weighted Average Variable Interest Rate (as a percent)					0.19%	0.16%
Swaps | Over 72 months to 84 months
Derivative Instruments
Aggregate notional amount of derivatives					$ 0	$ 200,000
Weighted Average Fixed-Pay Interest Rate (as a percent)					0.00%	2.20%
Weighted Average Variable Interest Rate (as a percent)					0.00%	0.17%
Swaps | Over 84 Months
Derivative Instruments
Aggregate notional amount of derivatives					$ 100,000	$ 100,000
Weighted Average Fixed-Pay Interest Rate (as a percent)					2.75%	2.75%
Weighted Average Variable Interest Rate (as a percent)					0.19%	0.16%
New Swaps | Swaps
Derivative Instruments
Aggregate notional amount	$ 0		$ 0
Weighted Average Fixed-Pay Interest Rate (as a percent)					0.00%
Number of new Swaps | derivative	0		0
Swaps amortized expired | Swaps
Derivative Instruments
Aggregate notional amount	$ 300,000		$ 710,170
Agency MBS, at fair value | Interest Rate Contract
Derivative Instruments
Assets pledged					$ 44,621	$ 57,247
Restricted cash | Interest Rate Contract
Derivative Instruments
Restricted cash					67,333	$ 66,486
Hedging | Swaps
Derivative Instruments
Aggregate notional amount of derivatives					3,050,000
Net unrealized gains (losses)	(64,600)		$ (64,600)		$ (64,600)
Average maturity term of swaps			51 months
Maximum maturity term of swaps			98 months
AOCI from derivative hedging instruments:
Balance at end of period	$ (64,600)		$ (64,600)
QTD | Swaps amortized expired | Swaps
Derivative Instruments
Weighted Average Fixed-Pay Interest Rate (as a percent)					2.06%
YTD | Swaps amortized expired | Swaps
Derivative Instruments
Weighted Average Fixed-Pay Interest Rate (as a percent)					1.96%