Derivative Instruments (Credit Exposure of Derivative Contracts) (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Credit Exposure of Derivative Contracts
Estimated fair value of derivatives in net asset (liability) position		$ 151,009	$ 236,985
Derivative, Collateral, Right to Reclaim Cash	[1]	30,771	1,441
Derivative Liability, Fair Value of Collateral	[2]	58,360	95,863
Derivative, Collateral, Obligation to Return Cash	[3]	(174,120)	(254,498)
Derivative Asset, Fair Value of Collateral	[4]	(17,782)	(16,913)
Deposits with Clearing Organizations and Others, Securities	[2]	(58,360)	(73,571)
Net credit exposure		(10,122)	(10,693)
Margin account related to exchange-traded futures	[5]	$ 7,832	$ 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] Included in cash and cash equivalents, with obligation to return cash collateral recorded in other liabilities.

[4] Consists of U.S. Treasury and government securities.

[5] Included in other assets.