Derivative Instruments (Details) - Summary of financial operations derivatives credit exposure (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Credit Derivatives [Line Items]
Total liability recorded	$ 20,366	$ 15,763
Weighted average years to maturity	16 years 1 month 6 days
Credit Exposure [Member] | Standard Poors BB Rating Member
Credit Derivatives [Line Items]
Principal outstanding	44,281	78,425
Interest outstanding	2,622	3,253
Aggregate outstanding exposure	46,903	81,678
Total liability recorded	$ 0	$ 10,288
Weighted average years to maturity	4 years 8 months 12 days	5 years