Derivatives (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
	Dec. 31, 2019	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		$ 107,801	$ (195,567)
Derivative Liability, Fair Value, Gross Liability		19,866		$ 1,634
Derivative assets, fair value		109,746		11,342
Derivative liabilities, fair value		(19,866)		(1,634)
Interest rate swaps
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Loss		0	(182,181)
Interest rate swaption
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		57,763
Derivative Instruments Not Designated as Hedging Instruments, Loss			(573)
Derivative instrument, cost basis		$ 6,688		13,000
Derivative Liability, Fair Value, Gross Liability				$ 107
Derivative, Average Fixed Interest Rate		1.16%		1.07%
Derivative Asset, Notional Amount		$ 500,000		$ 1,250,000
Derivative assets, fair value		27,843		1,253
U.S. Treasury futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		95,647
Derivative Instruments Not Designated as Hedging Instruments, Loss			(8,447)
Options on U.S. Treasury futures
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		12,617
Derivative Instruments Not Designated as Hedging Instruments, Loss			(10,727)
Long position | TBA securities
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain			18,432
Derivative Instruments Not Designated as Hedging Instruments, Loss		(58,226)
Derivative Liability, Fair Value, Gross Liability		(19,866)
Short position | U.S. Treasury futures
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability				1,527
Short position | TBA securities
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Loss		0	$ (12,071)
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption
Derivative [Line Items]
Derivative Asset, Notional Amount		500,000		1,250,000
Derivative assets, fair value		27,843		1,360
Derivative liabilities, fair value				(107)
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption | 6 months or less
Derivative [Line Items]
Derivative instrument, cost basis		$ 2,963		$ 6,312
Derivative, Average Fixed Interest Rate		1.12%		1.02%
Derivative Asset, Notional Amount		$ 250,000		$ 750,000
Derivative assets, fair value		14,208		1,161
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption | 6-9 months
Derivative [Line Items]
Derivative instrument, cost basis		$ 3,725		$ 6,688
Derivative, Average Fixed Interest Rate		1.19%		1.16%
Derivative Asset, Notional Amount		$ 250,000		$ 500,000
Derivative assets, fair value		13,635		92
Not Designated as Hedging Instrument, Economic Hedge | Options on U.S. Treasury futures
Derivative [Line Items]
Derivative Asset, Notional Amount		250,000		500,000
Derivative assets, fair value		$ 9,180		1,094
Derivative, Average Remaining Maturity	1 month	2 months
Not Designated as Hedging Instrument, Economic Hedge | Short position | U.S. Treasury futures
Derivative [Line Items]
Derivative Liability, Notional Amount		$ 2,980,000		825,000
Derivative assets, fair value		72,723
Derivative liabilities, fair value		$ 0		$ (1,527)
Derivative, Average Remaining Maturity		3 months		2 months
Not Designated as Hedging Instrument, Trading | Long position | TBA securities
Derivative [Line Items]
Derivative Asset, Notional Amount		$ 2,800,000		$ 1,515,000
Derivative assets, fair value				$ 8,888
Derivative liabilities, fair value		$ (19,866)