Derivatives Volume of Activity (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption
Derivative [Line Items]
Derivative, Notional Amount	$ (500,000)	$ (1,250,000)
Notional Amount of Derivative Instruments Added	0
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(750,000)
Not Designated as Hedging Instrument, Economic Hedge | U.S. Treasury futures | Short position
Derivative [Line Items]
Derivative, Notional Amount	(3,250,000)	(825,000)
Notional Amount of Derivative Instruments Added	(7,510,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(5,085,000)
Not Designated as Hedging Instrument, Economic Hedge | Options on U.S. Treasury futures | Long position
Derivative [Line Items]
Derivative, Notional Amount	(850,000)	(500,000)
Notional Amount of Derivative Instruments Added	(2,350,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(2,000,000)
Not Designated as Hedging Instrument, Trading | TBA securities | Long position
Derivative [Line Items]
Derivative, Notional Amount	(2,310,000)	$ (1,515,000)
Notional Amount of Derivative Instruments Added	(13,600,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	$ (12,805,000)