Derivative Instruments - Summary of financial operations derivatives credit exposure (Details) (Credit exposure, Standard & Poor's, CC Rating, USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Credit exposure | Standard & Poor's, CC Rating
Credit Derivatives [Line Items]
Principal outstanding	$ 42,043	$ 42,080
Interest outstanding	1,877	2,154
Aggregate outstanding exposure	43,920	44,234
Total liability recorded	$ 9,200	$ 4,190
Weighted average contractual term to maturity	3 years 6 months 0 days	3 years 8 months 12 days