Derivatives (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Derivative [Line Items]
Variation margin		$ (44,140)		$ (44,140)		$ 8,424
Derivative assets, fair value		4,845		4,845		6,563
Derivative liabilities, fair value		(439)		(439)		$ (1,218)
Gain (loss) on derivative instruments, net		$ (50,709)	$ 19,499	$ (229,941)	$ 78,520
Derivative, Swaption Interest Rate	[1]	2.07%		2.07%
Derivative, Term of Contract				10 years
Interest rate swaps
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain			25,019		93,833
Derivative Instruments Not Designated as Hedging Instruments, Loss		$ (52,908)		$ (248,886)
Derivative, Average Fixed Interest Rate, Current Effective		1.65%		1.65%		2.29%
Interest rate swaption
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Loss		$ (4,329)	0	$ (4,329)	0
Eurodollar futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		1,712		1,610	1,886
Derivative Instruments Not Designated as Hedging Instruments, Loss			(189)
U.S. Treasury futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Loss		0	0	(109)	0
Options on U.S. Treasury futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain					764
Derivative Instruments Not Designated as Hedging Instruments, Loss		0	(127)	0
Forward starting interest rate swap
Derivative [Line Items]
Derivative, Notional Amount		0		0		$ (775,000)
Long position | TBA securities
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		4,652		21,609
Derivative Instruments Not Designated as Hedging Instruments, Loss			(5,204)		(18,256)
Short position | TBA securities
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		164	$ 0	164	$ 293
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative assets, fair value	[2]	0		0		324
Derivative, Notional Amount	[3]	$ (3,880,000)		(3,880,000)		$ (4,515,000)
Notional Amount of Derivative Instruments Added				5,560,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off				$ (6,195,000)
Derivative, Average Fixed Interest Rate	[4]	1.65%		1.65%		2.35%
Derivative, Average Remaining Maturity				4 years 8 months 12 days	5 years 6 months
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaption
Derivative [Line Items]
Derivative instrument, cost basis		$ 6,180		$ 6,180
Derivative assets, fair value		1,851		1,851		$ 0
Derivative, Notional Amount		(750,000)		(750,000)		0
Notional Amount of Derivative Instruments Added				1,500,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off				(750,000)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Eurodollar futures
Derivative [Line Items]
Derivative liabilities, fair value		(928)		(928)		0
Derivative, Notional Amount		(6,000,000)		(6,000,000)		0
Notional Amount of Derivative Instruments Added				9,000,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off				(3,000,000)
Not Designated as Hedging Instrument, Economic Hedge [Member] | U.S. Treasury futures
Derivative [Line Items]
Derivative liabilities, fair value		0		0		(1,218)
Derivative, Notional Amount		0		0		(50,000)
Notional Amount of Derivative Instruments Added				0
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off				(50,000)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Short position | TBA securities
Derivative [Line Items]
Derivative liabilities, fair value		(439)		(439)		0
Derivative, Notional Amount		(500,000)		(500,000)		0
Notional Amount of Derivative Instruments Added				2,300,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off				(1,800,000)
Not Designated as Hedging Instrument, Trading | Long position | TBA securities
Derivative [Line Items]
Derivative assets, fair value		2,066		2,066		6,239
Derivative, Notional Amount		(395,000)		(395,000)		(860,000)
Notional Amount of Derivative Instruments Added				6,935,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off				(7,400,000)
Maturity in three years or less | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount	[3]	$ (2,360,000)		$ (2,360,000)		$ (1,560,000)
Derivative, Average Fixed Interest Rate		1.58%		1.58%		1.96%
Derivative, Average Remaining Maturity				1 year 7 months 6 days	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	[3]	$ (550,000)		$ (550,000)		$ (1,230,000)
Derivative, Average Fixed Interest Rate		1.35%		1.35%		2.23%
Derivative, Average Remaining Maturity				4 years 10 months 24 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	[3]	$ (850,000)		$ (850,000)		$ (1,505,000)
Derivative, Average Fixed Interest Rate		1.85%		1.85%		2.80%
Derivative, Average Remaining Maturity				9 years 7 months 6 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	[3]	$ (120,000)		$ (120,000)		$ (220,000)
Derivative, Average Fixed Interest Rate		2.84%		2.84%		2.81%
Derivative, Average Remaining Maturity				27 years 10 months 24 days	21 years 10 months 24 days

[1] Receive-variable rate based on 3-month LIBOR.

[2] Amounts shown as of September 30, 2019 and December 31, 2018 are net of $(44,140) paid and $8,424 received, respectively, in variation margin which is recorded on the Company’s consolidated balance sheets within “restricted cash.” As of September 30, 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] The notional amounts include $0 and $775,000 of forward starting pay-fixed interest rate swaps as of September 30, 2019 and December 31, 2018, respectively.

[4] Excluding forward starting pay-fixed interest rate swaps, the weighted average pay rate was 1.65% and 2.29% as of September 30, 2019 and December 31, 2018, respectively.