Derivatives Volume of Activity (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
Interest rate swaption
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 1,250,000
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaps
Derivative [Line Items]
Derivative Asset, Notional Amount	4,225,000
Notional Amount of Derivative Instruments Added	2,915,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(7,140,000)
Derivative Asset, Notional Amount	0
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption
Derivative [Line Items]
Derivative Asset, Notional Amount	750,000
Notional Amount of Derivative Instruments Added	1,250,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(750,000)
Derivative Asset, Notional Amount	1,250,000
Not Designated as Hedging Instrument, Economic Hedge | U.S. Treasury futures | Short position
Derivative [Line Items]
Derivative Liability, Notional Amount	0
Notional Amount of Derivative Instruments Added	5,737,600
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(4,912,600)
Derivative Liability, Notional Amount	825,000
Not Designated as Hedging Instrument, Economic Hedge | Options on U.S. Treasury futures
Derivative [Line Items]
Derivative Asset, Notional Amount	1,350,000
Notional Amount of Derivative Instruments Added	5,650,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(6,500,000)
Derivative Asset, Notional Amount	500,000
Not Designated as Hedging Instrument, Economic Hedge | TBA securities | Short position
Derivative [Line Items]
Derivative Liability, Notional Amount	500,000
Notional Amount of Derivative Instruments Added	3,017,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(3,517,000)
Derivative Liability, Notional Amount	0
Not Designated as Hedging Instrument, Trading | TBA securities | Long position
Derivative [Line Items]
Derivative Asset, Notional Amount	435,000
Notional Amount of Derivative Instruments Added	15,516,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(14,436,000)
Derivative Asset, Notional Amount	$ 1,515,000