Derivative Instruments (Credit Exposure of Derivative Contracts) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Credit Exposure of Derivative Contracts
Estimated fair value of derivatives in net asset (liability) position	$ 127,367		$ 59,016
Derivative, Collateral, Right to Reclaim Cash	0	[1]	8,033	[1]
Derivative Liability, Fair Value of Collateral	23,004	[2]	18,081	[2]
Derivative, Collateral, Obligation to Return Cash	(103,981)	[3]	(51,006)	[3]
Derivative Asset, Fair Value of Collateral	(19,532)	[4]	(11,095)	[4]
Deposits with Clearing Organizations and Others, Securities	(13,854)		(13,350)
Net credit exposure	13,004		9,679
Margin account related to exchange-traded futures	$ 6,524	[5]	$ 2,566	[5]

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

[2] Included in other invested assets, 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.