Derivative Instruments - Summary of financial operations derivatives credit exposure (Details) (Credit exposure, Standard & Poor's, CC Rating, USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014 Contracts	Dec. 31, 2013
Credit exposure | Standard & Poor's, CC Rating
Credit Derivatives [Line Items]
Derivative Liability, Number of Instruments Held	1
Principal outstanding	$ 35,789	$ 42,080
Interest outstanding	1,399	2,154
Aggregate outstanding exposure	37,188	44,234
Total liability recorded	$ 8,220	$ 4,190
Weighted average contractual term to maturity	3 years 0 months 0 days	3 years 8 months 12 days
Project Finance Loan Participation Deficiency Guarantee Percentage	68.00%