Derivatives (Details 1) - USD ($)		Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative, Notional Amount		$ 644,000,000	$ 969,000,000
Long [Member] | Float Interest Rate Swap Agreements [Member]
Derivative [Line Items]
Derivative, Notional Amount		644,000,000	969,000,000
Notional amount of TBAs [Member] | Long [Member]
Derivative [Line Items]
Derivative, Notional Amount			75,000,000
Notional amount of TBAs [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount		25,000,000
US Treasury Securities [Member] | Long [Member]
Derivative [Line Items]
Derivative, Notional Amount		0	226,000,000
US Treasury Securities [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount		24,000,000	0
US Treasury Futures [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount	[1]	$ 141,500,000	$ 0

[1] Each U.S. Treasury Future contract embodies $100,000 of notional value.