Derivatives Volume of Activity (Details) $ in Thousands	3 Months Ended
	Mar. 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	[1]
End of Period Notional Amount	5,015,000	[1]
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps | Short [Member]
Derivative [Line Items]
Beginning of Period Notional Amount	4,515,000
Notional Amount of Derivative Instruments Added	1,500,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(1,000,000)
End of Period Notional Amount	5,015,000
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	2,720,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(2,865,000)
End of Period Notional Amount	$ 715,000

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