Derivatives Volume of Activity (Details)	12 Months Ended
	Dec. 31, 2019 USD ($)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Beginning of Period, Notional Amount	$ 4,515,000,000
Notional Amount of Derivative Instruments Added	(6,560,000,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(6,850,000,000)
End of Period, Notional Amount	4,225,000,000
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaption
Derivative [Line Items]
Beginning of Period, Notional Amount	0
Notional Amount of Derivative Instruments Added	(1,500,000,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(750,000,000)
End of Period, Notional Amount	750,000,000
Not Designated as Hedging Instrument, Economic Hedge [Member] | Eurodollar futures
Derivative [Line Items]
Beginning of Period, Notional Amount	0
Notional Amount of Derivative Instruments Added	(9,000,000,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(9,000,000,000)
End of Period, Notional Amount	0
Not Designated as Hedging Instrument, Economic Hedge [Member] | U.S. Treasury futures
Derivative [Line Items]
Beginning of Period, Notional Amount	50,000,000
Notional Amount of Derivative Instruments Added	0
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(50,000,000)
End of Period, Notional Amount	0
Not Designated as Hedging Instrument, Economic Hedge [Member] | Options on U.S. Treasury futures
Derivative [Line Items]
Beginning of Period, Notional Amount	0
Notional Amount of Derivative Instruments Added	(1,350,000,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	0
End of Period, Notional Amount	1,350,000,000
Not Designated as Hedging Instrument, Economic Hedge [Member] | TBA securities | Short position
Derivative [Line Items]
Beginning of Period, Notional Amount	0
Notional Amount of Derivative Instruments Added	(1,800,000,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(2,300,000,000)
End of Period, Notional Amount	500,000,000
Not Designated as Hedging Instrument, Trading | TBA securities | Long position
Derivative [Line Items]
Beginning of Period, Notional Amount	860,000,000
Notional Amount of Derivative Instruments Added	(7,615,000,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(8,040,000,000)
End of Period, Notional Amount	$ 435,000,000