Derivatives Volume of Activity (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption
Derivative [Line Items]
Derivative, Notional Amount	$ (750,000)	$ (1,250,000)
Notional Amount of Derivative Instruments Added	(500,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(1,000,000)
Not Designated as Hedging Instrument, Economic Hedge | U.S. Treasury futures | Short position
Derivative [Line Items]
Derivative, Notional Amount	(3,540,000)	(825,000)
Notional Amount of Derivative Instruments Added	(11,640,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(8,925,000)
Not Designated as Hedging Instrument, Economic Hedge | Options on U.S. Treasury futures | Long position
Derivative [Line Items]
Derivative, Notional Amount	0	(500,000)
Notional Amount of Derivative Instruments Added	(2,350,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(2,850,000)
Not Designated as Hedging Instrument, Trading | TBA securities | Long position
Derivative [Line Items]
Derivative, Notional Amount	(1,980,000)	$ (1,515,000)
Notional Amount of Derivative Instruments Added	(20,235,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	$ (19,770,000)