Derivatives (Details) - USD ($)		3 Months Ended									12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Derivative [Line Items]
Derivative assets, fair value		$ 4,290,000				$ 6,563,000					$ 4,290,000	$ 6,563,000
Derivative liabilities, fair value		(974,000)				(1,218,000)					(974,000)	(1,218,000)
Variation margin		(9,265,000)				8,424,000					(9,265,000)	8,424,000
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		$ 42,992,000	$ (50,709,000)	$ (117,535,000)	$ (61,697,000)	(81,981,000)		$ 19,499,000	$ 20,667,000	$ 38,354,000	$ (186,949,000)	(3,461,000)		$ 3,044,000
Derivative, Swaption Interest Rate	[1]	2.07%									2.07%
Interest rate swaps
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain												10,363,000
Derivative Instruments Not Designated as Hedging Instruments, Loss											$ (202,450,000)			(2,632,000)
Interest rate swaption
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Loss											$ (5,607,000)	0		0
Derivative, Maturity Date											Jan. 16, 2020
Eurodollar futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain											$ 2,359,000	1,887,000		821,000
U.S. Treasury futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Loss											(109,000)	(4,609,000)		0
Options on U.S. Treasury futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Loss											(1,422,000)	(658,000)		0
Long position | TBA securities
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain											20,020,000			5,757,000
Derivative Instruments Not Designated as Hedging Instruments, Loss												(10,737,000)
Short position | TBA securities
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain											260,000	293,000
Derivative Instruments Not Designated as Hedging Instruments, Loss														$ (902,000)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative assets, fair value	[2]	$ 0				324,000					0	324,000
Derivative, Notional Amount		$ 4,225,000,000				$ 4,515,000,000					$ 4,225,000,000	$ 4,515,000,000
Derivative, Average Fixed Interest Rate		1.62%				2.35%	[3]				1.62%	2.35%	[3]
Derivative, Average Remaining Maturity											3 years 9 months 18 days	5 years 6 months
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaption
Derivative [Line Items]
Derivative assets, fair value						$ 0						$ 0
Derivative, Notional Amount		$ 750,000,000				0					$ 750,000,000	0
Derivative instrument, cost basis		6,180,000									6,180,000
Not Designated as Hedging Instrument, Economic Hedge [Member] | Eurodollar futures
Derivative [Line Items]
Derivative, Notional Amount		0				0					0	0
Not Designated as Hedging Instrument, Economic Hedge [Member] | U.S. Treasury futures
Derivative [Line Items]
Derivative liabilities, fair value		0				(1,218,000)					0	(1,218,000)
Derivative, Notional Amount		0				50,000,000					0	50,000,000
Not Designated as Hedging Instrument, Economic Hedge [Member] | Options on U.S. Treasury futures
Derivative [Line Items]
Derivative assets, fair value						0						0
Derivative, Notional Amount		1,350,000,000				0					$ 1,350,000,000	0
Derivative, Maturity Date											Feb. 21, 2020
Derivative instrument, cost basis		4,359,000									$ 4,359,000
Not Designated as Hedging Instrument, Economic Hedge [Member] | Short position | TBA securities
Derivative [Line Items]
Derivative liabilities, fair value						0						0
Derivative, Notional Amount		500,000,000				0					500,000,000	0
Not Designated as Hedging Instrument, Trading | Long position | TBA securities
Derivative [Line Items]
Derivative assets, fair value						6,239,000						6,239,000
Derivative, Notional Amount		435,000,000				860,000,000					435,000,000	860,000,000
Maturity in three years or less | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount		$ 2,860,000,000				$ 1,560,000,000					$ 2,860,000,000	$ 1,560,000,000
Derivative, Average Fixed Interest Rate		1.58%				1.96%					1.58%	1.96%
Derivative, Average Remaining Maturity											1 year 6 months	1 year 4 months 24 days
Maturity between 3 and 6 years | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount		$ 700,000,000				$ 1,230,000,000					$ 700,000,000	$ 1,230,000,000
Derivative, Average Fixed Interest Rate		1.43%				2.23%					1.43%	2.23%
Derivative, Average Remaining Maturity											4 years 8 months 12 days	4 years 4 months 24 days
Maturity between 6 and 10 years | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount		$ 545,000,000				$ 1,505,000,000					$ 545,000,000	$ 1,505,000,000
Derivative, Average Fixed Interest Rate		1.78%				2.80%					1.78%	2.80%
Derivative, Average Remaining Maturity											9 years 4 months 24 days	8 years 3 months 18 days
Maturity greater than 10 years [Member] | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount		$ 120,000,000				$ 220,000,000					$ 120,000,000	$ 220,000,000
Derivative, Average Fixed Interest Rate		2.84%				2.81%					2.84%	2.81%
Derivative, Average Remaining Maturity											27 years 8 months 12 days	21 years 10 months 24 days

[1] Weighted average based on notional amount of the underlying instrument.

[2] Amounts shown as of December 31, 2019 and December 31, 2018 are net of $(9,265) paid and $8,424 received, respectively, in variation margin which is recorded on the Company’s consolidated balance sheets within “restricted cash.” As of December 31, 2019, all 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. The amount shown as of December 31, 2018 is the unsettled fair value of the instruments subject to bilateral agreements and not centrally cleared through the CME as of that date.

[3] Implied cost basis represents the forward price to be paid for the underlying Agency MBS as if settled as of the date indicated.