Derivatives Volume of Activity (Details)	3 Months Ended
	Mar. 31, 2020 USD ($)
Derivative [Line Items]
Derivative Liability, Notional Amount	$ 1,177,500,000
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaps
Derivative [Line Items]
Derivative Asset, Notional Amount	4,225,000,000
Notional Amount of Derivative Instruments Added	(2,570,000,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(6,565,000,000)
Derivative Asset, Notional Amount	230,000,000
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption
Derivative [Line Items]
Derivative Asset, Notional Amount	750,000,000	[1]
Notional Amount of Derivative Instruments Added	0
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(750,000,000)
Derivative Asset, Notional Amount	0
Not Designated as Hedging Instrument, Economic Hedge | U.S. Treasury futures
Derivative [Line Items]
Derivative Liability, Notional Amount	0
Notional Amount of Derivative Instruments Added	(1,477,500,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(300,000,000)
Derivative Liability, Notional Amount	1,177,500,000
Not Designated as Hedging Instrument, Economic Hedge | Options on U.S. Treasury futures
Derivative [Line Items]
Derivative Asset, Notional Amount	1,350,000,000	[1]
Notional Amount of Derivative Instruments Added	(2,000,000,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(1,650,000,000)
Derivative Asset, Notional Amount	1,700,000,000	[2]
Not Designated as Hedging Instrument, Economic Hedge | TBA securities | Short position
Derivative [Line Items]
Derivative Liability, Notional Amount	500,000,000
Notional Amount of Derivative Instruments Added	(830,000,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(815,000,000)
Derivative Liability, Notional Amount	515,000,000
Not Designated as Hedging Instrument, Trading | TBA securities | Long position
Derivative [Line Items]
Derivative Asset, Notional Amount	435,000,000
Notional Amount of Derivative Instruments Added	(1,905,000,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(2,340,000,000)
Derivative Asset, Notional Amount	$ 0

[1] All options outstanding as of December 31, 2019 expired during the first quarter of 2020.

[2] All futures and options outstanding as of March 31, 2020 will expire in June of 2020