Derivative Instruments (Credit Exposure of Derivative Contracts) (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Credit Exposure of Derivative Contracts
Estimated fair value of derivatives in net asset (liability) position		$ 252,059	$ 175,209
Derivative, Collateral, Right to Reclaim Cash	[1]	15,916	0
Derivative Liability, Fair Value of Collateral	[2]	60,120	47,222
Derivative, Collateral, Obligation to Return Cash	[3]	(260,135)	(178,141)
Derivative Asset, Fair Value of Collateral	[4]	(19,127)	(20,260)
Deposits with Clearing Organizations and Others, Securities		(33,380)	(16,333)
Net credit exposure		15,453	7,697
Margin account related to exchange-traded futures	[5]	$ 9,761	$ 7,976

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

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

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

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

[5] Included in cash and cash equivalents.