Derivative Instruments (Credit Exposure of Derivative Contracts) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Credit Exposure of Derivative Contracts
Estimated fair value of derivatives in net asset (liability) position	$ 74,273		$ 136,558
Derivative, Collateral, Right to Reclaim Cash	8,150	[1]	27,867	[1]
Derivative Liability, Fair Value of Collateral	15,238	[2]	1,565	[2]
Derivative, Collateral, Obligation to Return Cash	(64,027)	[3]	(136,414)	[3]
Derivative Asset, Fair Value of Collateral	(10,624)	[4]	(22,458)	[4]
Deposits with Clearing Organizations and Others, Securities	(10,869)		0
Net credit exposure	12,141		7,118
Margin account related to exchange-traded futures	$ 3,833	[5]	$ 5,605	[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.