(Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivative [Line Items]
Derivative assets, fair value		$ 7,642		$ 7,642		$ 2,940
Gain (loss) on derivative instruments, net		$ 20,667	$ (15,802)	$ 59,021	$ (15,627)
Derivative, Average Fixed Interest Rate, Current Effective		1.85%		1.85%		1.36%
Other Footnotes Disclosing Derivative-Related Information, Cross-Reference				Note 1
Forward starting interest rate swap
Derivative [Line Items]
Derivative, Notional Amount		$ (1,575,000)		$ (1,575,000)		$ (2,525,000)
Eurodollar futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		170	0	2,075	0
Options on U.S. Treasury futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		891	0	891	0
Long [Member] | Interest rate swaps
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain			979		845
Derivative Instruments Not Designated as Hedging Instruments, Loss		(376)		(1,505)
Long [Member] | TBA securities
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain			1,717		1,717
Derivative Instruments Not Designated as Hedging Instruments, Loss		(965)		(13,052)
Short [Member] | Interest rate swaps
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		20,947		70,319
Derivative Instruments Not Designated as Hedging Instruments, Loss			(18,498)		(18,189)
Short [Member] | TBA securities
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		0	$ 0	293	$ 0
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative assets, fair value		985		985		791
Derivative, Notional Amount	[1]	$ (3,850,000)		$ (3,850,000)		$ (5,675,000)
Derivative, Average Fixed Interest Rate	[2]	2.07%		2.07%		1.71%
Derivative, Average Remaining Maturity				5 years 2 months 9 days		3 years 1 month 6 days
Derivative, Fair Value, Net	[3]	$ 985		$ 985		$ 791
Not Designated as Hedging Instrument, Economic Hedge [Member] | Eurodollar futures
Derivative [Line Items]
Derivative assets, fair value		967		967		666
Derivative, Notional Amount	[4]	(650,000)		(650,000)		(1,950,000)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Long [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount		(100,000)		(100,000)		(100,000)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Short [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount		(3,950,000)		(3,950,000)		(5,775,000)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Short [Member] | TBA securities
Derivative [Line Items]
Derivative liabilities, fair value		0		0		269
Derivative, Notional Amount	[5]	0		0		(150,000)
Not Designated as Hedging Instrument, Trading | Options on U.S. Treasury futures
Derivative [Line Items]
Derivative assets, fair value		3,656		3,656		0
Derivative, Notional Amount		(300,000)		(300,000)		0
Not Designated as Hedging Instrument, Trading | Long [Member] | TBA securities
Derivative [Line Items]
Derivative assets, fair value		2,034		2,034		1,483
Derivative, Notional Amount	[5]	(765,000)		(765,000)		(795,000)
Maturity in three years or less | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount	[1]	$ (1,220,000)		$ (1,220,000)		$ (3,320,000)
Derivative, Average Fixed Interest Rate		1.63%		1.63%		1.35%
Derivative, Average Remaining Maturity				1 year 3 months 11 days		8 months 12 days
Derivative, Fair Value, Net	[3]	$ 985		$ 985		$ 791
Maturity between 3 and 6 years | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount	[1]	$ (1,310,000)		$ (1,310,000)		$ (1,210,000)
Derivative, Average Fixed Interest Rate		2.01%		2.01%		2.00%
Derivative, Average Remaining Maturity				4 years 2 months 19 days		4 years 7 months 6 days
Derivative, Fair Value, Net	[3]	$ 0		$ 0		$ 0
Maturity between 6 and 10 years | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount	[1]	$ (1,100,000)		$ (1,100,000)		$ (1,025,000)
Derivative, Average Fixed Interest Rate		2.56%		2.56%		2.49%
Derivative, Average Remaining Maturity				7 years 11 months 16 days		8 years
Derivative, Fair Value, Net	[3]	$ 0		$ 0		$ 0
Maturity greater than 10 years [Member] | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount	[1]	$ (220,000)		$ (220,000)		$ (120,000)
Derivative, Average Fixed Interest Rate		2.81%		2.81%		2.75%
Derivative, Average Remaining Maturity				22 years 4 months 28 days		17 years 3 months 18 days
Derivative, Fair Value, Net	[3]	$ 0		$ 0		$ 0

[1] The net notional amounts included in the tables above represent pay-fixed interest rate swaps, net of receive-fixed interest rate swaps and include $1,575,000 and $2,525,000 of pay-fixed forward starting interest rate swaps as of June 30, 2018 and December 31, 2017, respectively.

[2] Excluding forward starting pay-fixed interest rate swaps, the weighted average pay rate was 1.85% and 1.36% as of June 30, 2018 and December 31, 2017, respectively.

[3] The majority of the Company’s interest rate swap agreements are centrally cleared through the CME. Please refer to Note 1 for information regarding the exchange of variation margin being legally considered as settlement of the derivative as opposed to a pledge of collateral.

[4] The Eurodollar futures notional amounts represent the total notional of the 3-month contracts all of which expire in 2018. The maximum notional outstanding for any future 3-month period did not exceed $650,000 during the period indicated.

[5] Notional amount represents the par value (or principal balance) of the underlying Agency MBS as if settled as of the end of the period.