Derivative Instruments (Derivative Instruments Offsetting Balance Sheet) (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Derivative Asset [Abstract]
Derivative Fair Value Of Free Standing Derivative Asset		$ 283,094	$ 301,270
Derivative Asset, Fair Value, Gross Liability		(27,028)	(30,096)
Derivative Asset		256,066	271,174
Derivative Asset, Fair Value of Collateral	[1],[2]	(16,913)	(20,888)
Derivative, Collateral, Obligation to Return Cash	[3]	(254,498)	(245,038)
Derivative Asset, Fair Value, Amount Offset Against Collateral		(15,345)	5,248
Derivative Liability [Abstract]
Derivative Fair Value Of Free Standing Derivative Liability		48,571	54,921
Derivative Liability, Fair Value, Gross Asset		(27,028)	(30,096)
Derivative Liability		21,543	24,825
Derivative Liability, Fair Value of Collateral	[2],[4]	(95,863)	(47,149)
Derivative, Collateral, Right to Reclaim Cash	[5]	(1,441)	(12,540)
Derivative Liability, Fair Value, Amount Offset Against Collateral		$ (75,761)	$ (34,864)

[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.