Derivatives Volume of Activity (Details)	6 Months Ended
	Jun. 30, 2020 USD ($)
Derivative [Line Items]
Derivative Liability, Notional Amount	$ 1,225,000,000	[1]
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	2,915,000,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(6,665,000,000)
Derivative Asset, Notional Amount	475,000,000
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption
Derivative [Line Items]
Derivative Asset, Notional Amount	750,000,000	[2]
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	2,212,600,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(3,437,600,000)
Derivative Liability, Notional Amount	1,225,000,000
Not Designated as Hedging Instrument, Economic Hedge | Options on U.S. Treasury futures
Derivative [Line Items]
Derivative Asset, Notional Amount	1,350,000,000	[2]
Notional Amount of Derivative Instruments Added	3,425,000,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(3,350,000,000)
Derivative Asset, Notional Amount	1,425,000,000	[1]
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	3,017,000,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(3,517,000,000)
Derivative Liability, Notional Amount	0
Not Designated as Hedging Instrument, Trading | TBA securities | Long position
Derivative [Line Items]
Derivative Asset, Notional Amount	435,000,000
Notional Amount of Derivative Instruments Added	5,311,000,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(4,496,000,000)
Derivative Asset, Notional Amount	$ 1,250,000,000

[1] All futures and options outstanding as of June 30, 2020 will expire in September of 2020

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