Derivative Instruments - Summary of financial operations derivatives credit exposure (Details) (Credit exposure, Standard & Poor's, BB Rating [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Credit exposure | Standard & Poor's, BB Rating [Member]
Credit Derivatives [Line Items]
Principal outstanding	$ 41,690	$ 44,281
Interest outstanding	2,308	2,622
Aggregate outstanding exposure	43,998	46,903
Total liability recorded	$ 0	$ 0
Weighted average contractual term to maturity	4 years 3 months 18 days	4 years 8 months 12 days