Derivative Instruments (Credit Exposure of Derivative Contracts) (Details) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015
Credit Exposure of Derivative Contracts
Estimated fair value of derivatives in net asset (liability) position		$ 285,180	$ 248,968
Derivative, Collateral, Right to Reclaim Cash	[1]	9,409	12,540
Derivative Liability, Fair Value of Collateral	[2]	74,663	47,149
Derivative, Collateral, Obligation to Return Cash	[3]	(265,257)	(245,038)
Derivative Asset, Fair Value of Collateral	[4]	(32,953)	(20,888)
Deposits with Clearing Organizations and Others, Securities		(65,036)	(34,898)
Net credit exposure		6,006	7,833
Margin account related to exchange-traded futures	[5]	$ 12,764	$ 11,004

[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 securities.

[5] Included in other assets.