Derivatives Volume of Activity (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
Interest rate swaption
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 1,250,000
Derivative Asset, Notional Amount	500,000
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption
Derivative [Line Items]
Derivative Asset, Notional Amount	1,250,000
Notional Amount of Derivative Instruments Added	0
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	750,000
Derivative Asset, Notional Amount	500,000
Not Designated as Hedging Instrument, Economic Hedge | U.S. Treasury futures | Short position
Derivative [Line Items]
Derivative Liability, Notional Amount	825,000
Notional Amount of Derivative Instruments Added	(4,010,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	1,855,000
Derivative Liability, Notional Amount	2,980,000
Not Designated as Hedging Instrument, Economic Hedge | Options on U.S. Treasury futures
Derivative [Line Items]
Derivative Asset, Notional Amount	500,000
Notional Amount of Derivative Instruments Added	(1,500,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	1,750,000
Derivative Asset, Notional Amount	250,000
Not Designated as Hedging Instrument, Trading | TBA securities | Long position
Derivative [Line Items]
Derivative Asset, Notional Amount	1,515,000
Notional Amount of Derivative Instruments Added	(8,065,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	6,780,000
Derivative Asset, Notional Amount	$ 2,800,000