Derivatives Volume of Activity (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps
Derivative [Line Items]
Beginning of Period Notional Amount	$ (4,515,000)	[1]
Notional Amount of Derivative Instruments Added	(5,560,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(6,195,000)
End of Period Notional Amount	(3,880,000)	[1]
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)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(750,000)
End of Period Notional Amount	(750,000)
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)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(3,000,000)
End of Period Notional Amount	(6,000,000)
Not Designated as Hedging Instrument, Economic Hedge [Member] | U.S. Treasury futures
Derivative [Line Items]
Beginning of Period Notional Amount	(50,000)
Notional Amount of Derivative Instruments Added	0
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(50,000)
End of Period Notional Amount	0
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	(2,300,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(1,800,000)
End of Period Notional Amount	(500,000)
Not Designated as Hedging Instrument, Trading | TBA securities | Long position
Derivative [Line Items]
Beginning of Period Notional Amount	(860,000)
Notional Amount of Derivative Instruments Added	(6,935,000)
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(7,400,000)
End of Period Notional Amount	$ (395,000)

[1] The notional amounts include $0 and $775,000 of forward starting pay-fixed interest rate swaps as of September 30, 2019 and December 31, 2018, respectively.