Derivative Instruments (Credit Exposure of Derivative Contracts) (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Credit Exposure of Derivative Contracts
Estimated fair value of derivatives in net asset (liability) position		$ 171,169	$ 155,714
Derivative, Collateral, Right to Reclaim Cash	[1]	3,470	22,221
Derivative Liability, Fair Value of Collateral	[2]	57,456	58,156
Derivative, Collateral, Obligation to Return Cash	[3]	(188,407)	(185,900)
Derivative Asset, Fair Value of Collateral	[4]	0	(862)
Deposits with Clearing Organizations and Others, Securities	[2]	(57,456)	(58,156)
Net credit exposure		(13,768)	(8,827)
Margin account related to exchange-traded futures	[5]	$ 7,382	$ 6,538

[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.