Derivatives (Details 1) - USD ($)		Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative, Notional Amount		$ 2,227,000,000	$ 644,000,000
Long [Member] | Float Interest Rate Swap Agreements [Member]
Derivative [Line Items]
Derivative, Notional Amount		2,227,000,000	644,000,000
Notional Amount of Swaptions [Member] | Long [Member]
Derivative [Line Items]
Derivative, Notional Amount		270,000,000	0
Net notional amount of TBAs [Member] | Long [Member]
Derivative [Line Items]
Derivative, Notional Amount		100,000,000	(25,000,000)
US Treasury Securities [Member] | Short [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount		(24,668,000)	(24,000,000)
Eurodollar Futures [Member] | Short [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	[1]	$ (52,500,000)	$ (141,500,000)

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