Derivative Instruments (Credit Exposure of Derivative Contracts) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Credit Exposure of Derivative Contracts [Abstract]
Derivative Assets (Liabilities), at Fair Value, Net		$ 155,714	$ 236,985
Derivative, Collateral, Right to Reclaim Cash	[1]	22,221	1,441
Derivative Liability, Fair Value of Collateral	[2],[3]	58,156	95,863
Derivative, Collateral, Obligation to Return Cash	[4]	(185,900)	(254,498)
Derivative Asset, Fair Value of Collateral	[3],[5]	(862)	(16,913)
Deposits with Clearing Organizations and Others, Securities		(58,156)	(73,571)
Net Credit Exposure On Derivative Contracts		(8,827)	(10,693)
Margin Deposit Assets	[6]	$ 6,538	$ 9,687

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

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

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

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

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

[6] Included in other assets.