Derivatives (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended			12 Months Ended
		Jun. 30, 2017		Jun. 30, 2016	Jun. 30, 2017		Jun. 30, 2016	Dec. 31, 2016
Derivative [Line Items]
Derivative assets, fair value		$ 267			$ 267			$ 28,534	[1]
Derivative liabilities, fair value		(1,686)			(1,686)			(6,922)	[1]
Derivative, Notional Amount	[2]	$ 5,285,000			$ 5,285,000			$ 3,030,000
Derivative, Average Fixed Interest Rate	[3]	1.67%			1.67%			1.58%
Derivative, Average Remaining Maturity					3 years 4 months 13 days			5 years 3 months 18 days
Derivative, Fair Value, Net		$ 249			$ 249			$ 21,612
Loss on derivative instruments, net		$ (15,802)		$ (16,297)	$ (15,627)		$ (64,561)
Derivative, Average Fixed Interest Rate, Current Effective		1.31%			1.31%			0.73%
Interest Rate Swap
Derivative [Line Items]
Derivative assets, fair value		$ 267			$ 267			$ 28,534	[1]
Derivative liabilities, fair value		(18)			(18)			(6,922)	[1]
Forward starting interest rate swap
Derivative [Line Items]
Derivative, Notional Amount		2,425,000			2,425,000			2,725,000
TBA security
Derivative [Line Items]
Derivative liabilities, fair value		(1,668)			(1,668)			0
Derivative, Notional Amount		400,000	[4]		400,000	[4]		0
Cost Basis,TBA	[5]	416,312			416,312
Market value, TBA	[6]	414,644			414,644
Net carrying value, TBA	[7]	(1,668)			(1,668)
Derivative Instruments Not Designated as Hedging Instruments, Gain		1,717		0	1,717		0
Eurodollar Future
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Loss		0		(4,186)	0		(16,372)
Receive-fixed interest rate swap | Interest Rate Swap
Derivative [Line Items]
Derivative, Notional Amount		100,000			100,000			425,000
Derivative Instruments Not Designated as Hedging Instruments, Gain		979		3,743	845		14,277
Pay-fixed interest rate swap | Interest Rate Swap
Derivative [Line Items]
Derivative, Notional Amount		5,385,000			5,385,000			3,455,000
Derivative Instruments Not Designated as Hedging Instruments, Loss		(18,498)		$ (15,854)	(18,189)		$ (62,466)
Maturity in three years or less
Derivative [Line Items]
Derivative, Notional Amount	[2]	$ 3,060,000			$ 3,060,000			$ 595,000
Derivative, Average Fixed Interest Rate		1.39%			1.39%			0.73%
Derivative, Average Remaining Maturity					1 year 1 month 6 days			2 years 3 months 18 days
Derivative, Fair Value, Net		$ 249			$ 249			$ 4,348
Maturity between 3 and 6 years
Derivative [Line Items]
Derivative, Notional Amount	[2]	$ 1,050,000			$ 1,050,000			$ 1,185,000
Derivative, Average Fixed Interest Rate		1.63%			1.63%			1.47%
Derivative, Average Remaining Maturity					4 years 3 months 18 days			4 years 3 months 18 days
Derivative, Fair Value, Net		$ 0			$ 0			$ 8,631
Maturity between 6 and 10 years
Derivative [Line Items]
Derivative, Notional Amount	[2]	$ 1,175,000			$ 1,175,000			$ 1,250,000
Derivative, Average Fixed Interest Rate		2.45%			2.45%			2.42%
Derivative, Average Remaining Maturity					8 years 3 months 29 days			8 years 10 months 24 days
Derivative, Fair Value, Net		$ 0			$ 0			$ 8,633
TBA 3 percent coupon | TBA security
Derivative [Line Items]
Derivative, Notional Amount	[4]	100,000			100,000
Cost Basis,TBA	[5]	100,656			100,656
Market value, TBA	[6]	99,867			99,867
Net carrying value, TBA	[7]	(789)			(789)
TBA 4 percent coupon | TBA security
Derivative [Line Items]
Derivative, Notional Amount	[4]	300,000			300,000
Cost Basis,TBA	[5]	315,656			315,656
Market value, TBA	[6]	314,777			314,777
Net carrying value, TBA	[7]	$ (879)			$ (879)

[1] Refer to Note 1 regarding information on a change in the CME rulebook. Amounts reported on the consolidated balance sheet as of June 30, 2017 for its interest rate swaps reflect the netting of the derivative asset or liability with the related collateral received or posted, respectively. The net amounts comparable to June 30, 2017 for the derivative asset and derivative liabilities as of December 31, 2016 were $104 and $(576), respectively.

[2] The net notional amounts included in the tables above represent pay-fixed interest rate swaps, net of receive-fixed interest rate swaps and include $2,425,000 and $2,725,000 of pay-fixed forward starting interest rate swaps as of June 30, 2017 and December 31, 2016, respectively.

[3] Excluding forward starting pay-fixed interest rate swaps, the weighted average pay rate was 1.31% and 0.73% as of June 30, 2017 and December 31, 2016, respectively.

[4] Notional amount represents the par value (or principal balance) of the underlying Agency MBS.

[5] Cost basis represents the forward price to be paid for the underlying Agency MBS as if settled.

[6] Market value is the current fair value of the TBA contract and represents the estimated fair value of the underlying Agency security as of the end of the period.

[7] Net carrying value represents the difference between the market value and the cost basis of the TBA contract as of the end of the period and is included on the consolidated balance sheets within "derivative assets (liabilities)".