Derivative Instruments (Derivative Instruments Offsetting Balance Sheet) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Derivative Asset [Abstract]
Derivative Fair Value Of Free Standing Derivative Asset		$ 198,530	$ 283,094
Derivative Asset, Fair Value, Gross Liability		(20,258)	(27,028)
Derivative Asset		178,272	256,066
Derivative Asset, Fair Value of Collateral	[1],[2]	(862)	(16,913)
Derivative, Collateral, Obligation to Return Cash	[3]	(185,900)	(254,498)
Derivative Asset, Fair Value, Amount Offset Against Collateral		(8,490)	(15,345)
Derivative Liability [Abstract]
Derivative Fair Value Of Free Standing Derivative Liability		44,499	48,571
Derivative Liability, Fair Value, Gross Asset		(20,258)	(27,028)
Derivative Liability		24,241	21,543
Derivative Liability, Fair Value of Collateral	[2],[4]	(58,156)	(95,863)
Derivative, Collateral, Right to Reclaim Cash	[5]	(22,221)	(1,441)
Derivative Liability, Fair Value, Amount Offset Against Collateral		$ (56,136)	$ (75,761)

[1] Consists of U.S. Treasury and government agency securities.

[2] Includes initial margin posted to a central clearing partner.

[3] Included in cash and cash equivalents, with obligation to return cash collateral recorded in other liabilities.

[4] Included in available-for-sale securities, primarily consists of U.S. Treasury and government agency securities.

[5] Consists of receivable from counterparty, included in other assets.