Derivative Instruments - Summary of financial operations derivatives credit exposure (Details) (Credit exposure, USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Sep. 30, 2013 Standard & Poor's, CC Rating	Dec. 31, 2012 Standard & Poor's, BB Rating
Credit Derivatives [Line Items]
Principal outstanding	$ 41,364	$ 44,281
Interest outstanding	2,118	2,622
Aggregate outstanding exposure	43,482	46,903
Total liability recorded	$ 4,190	$ 0
Weighted average contractual term to maturity	4 years 0 months 0 days	4 years 8 months 12 days