Derivative Instruments (Credit Exposure of Derivative Contracts) (Details) - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015
Credit Exposure of Derivative Contracts
Estimated fair value of derivatives in net asset (liability) position		$ 313,817	$ 248,968
Derivative, Collateral, Right to Reclaim Cash	[1]	20,078	12,540
Derivative Liability, Fair Value of Collateral	[2]	74,680	47,149
Derivative, Collateral, Obligation to Return Cash	[3]	(305,658)	(245,038)
Derivative Asset, Fair Value of Collateral	[4]	(48,024)	(20,888)
Deposits with Clearing Organizations and Others, Securities	[2]	(63,056)	(34,898)
Net credit exposure		(8,163)	7,833
Margin account related to exchange-traded futures	[5]	$ 10,101	$ 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.

[5] Included in other assets.