Derivatives Volume of Activity (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
TBA security
Derivative [Line Items]
Beginning of Period Notional Amount	$ 0
Notional Amount of Derivative Instruments Added	3,814,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(3,164,000)
End of Period Notional Amount	650,000
Interest Rate Swap | Receive-fixed interest rate swap
Derivative [Line Items]
Beginning of Period Notional Amount	425,000
Notional Amount of Derivative Instruments Added	0
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(325,000)
End of Period Notional Amount	100,000
Interest Rate Swap | Pay-fixed interest rate swap
Derivative [Line Items]
Beginning of Period Notional Amount	3,455,000
Notional Amount of Derivative Instruments Added	3,010,000
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off	(920,000)
End of Period Notional Amount	$ 5,545,000