Derivatives Volume of Activity (Details) $ in Thousands	6 Months Ended
	Jun. 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	2,955,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(3,970,000)
End of Period Notional Amount	3,500,000	[1]
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	3,000,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	0
End of Period Notional Amount	3,000,000
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, Trading | TBA securities | Long [Member]
Derivative [Line Items]
Beginning of Period Notional Amount	860,000
Notional Amount of Derivative Instruments Added	4,830,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(5,320,000)
End of Period Notional Amount	$ 370,000

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