Variable Interest Entities - Carry Amounts and Classification of Assets and Liabilities (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) entity	Jun. 30, 2016 USD ($) power_plant	Feb. 24, 2016 power_plant MW	Dec. 31, 2015 USD ($)
Variable interest entities
Variable Interest Entity [Line Items]
Number of variable interest entities, primary beneficiary	13	11
VIE, primary beneficiary
Variable Interest Entity [Line Items]
Total assets	$ 723,604	$ 723,604		$ 772,523
Total liabilities	459,149	459,149		431,712
VIE, primary beneficiary | Current assets
Variable Interest Entity [Line Items]
Total assets	194,980	194,980		212,295
VIE, primary beneficiary | Non-current assets
Variable Interest Entity [Line Items]
Total assets	528,624	528,624		560,228
VIE, primary beneficiary | Current liabilities
Variable Interest Entity [Line Items]
Total liabilities	397,705	397,705		351,554
VIE, primary beneficiary | Non-current liabilities
Variable Interest Entity [Line Items]
Total liabilities	$ 61,444	$ 61,444		$ 80,158
NPS Star and WXA
Variable Interest Entity [Line Items]
Percentage of voting interests acquired			49.00%
Number of projects acquired | power_plant			2
Capacity of acquired power plants | MW			35.6
NPS Star
Variable Interest Entity [Line Items]
Number of projects acquired | power_plant			3
Capacity of acquired power plants | MW			17.8
WXA
Variable Interest Entity [Line Items]
Number of projects acquired | power_plant			3
Capacity of acquired power plants | MW			17.8