Acquisitions - Additional Information (Details) $ in Thousands			3 Months Ended
	Mar. 07, 2018 USD ($) MW	Apr. 21, 2017 USD ($) kV mi MW	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)
Business Acquisition [Line Items]
Liabilities assumed			$ 352,570		$ 148,456
Long-term debt, gross			2,224,704	$ 1,967,627
Tsugaru Holdings [Member]
Business Acquisition [Line Items]
Consideration	$ 282,548
Cash Consideration	176,600
Contingent Consideration	105,900
Liabilities assumed	181,300
Transaction costs			1,300
Decrease in property, plant and equipment			600
Decrease in construction in progress			$ 600
Debt Assumed	181,300
Tsugaru Holdings [Member] | Term Conversion Of Tsugaru Construction Loan [Member]
Business Acquisition [Line Items]
Contingent Consideration	102,900
Tsugaru Holdings [Member] | Term Conversion Of Ohorayama Construction Loan [Member]
Business Acquisition [Line Items]
Contingent Consideration	$ 3,000
Broadview Project [Member]
Business Acquisition [Line Items]
Cash Consideration		$ 214,700
Contingent Consideration		$ 21,300
Percentage of voting interests acquired		100.00%
Consideration transferred, liabilities incurred		$ 2,400
Debt Assumed		$ 51,200
Broadview Holdings' Interest in Broadview [Member]
Business Acquisition [Line Items]
Ownership percentage		100.00%
Tsugaru [Member] | Aomori Prefecture, Japan [Member] | Tsugaru Holdings [Member]
Business Acquisition [Line Items]
Generating capacity | MW	122
Ohorayama [Member] | Kochi Prefecture, Japan [Member] | Tsugaru Holdings [Member]
Business Acquisition [Line Items]
Generating capacity | MW	33
Kanagi [Member] | Shimane Prefecture, Japan [Member] | Tsugaru Holdings [Member]
Business Acquisition [Line Items]
Generating capacity | MW	10
Otsuki [Member] | Kochi Prefecture, Japan [Member] | Tsugaru Holdings [Member]
Business Acquisition [Line Items]
Generating capacity | MW	12
Futtsu [Member] | Chiba Prefecture, Japan [Member] | Tsugaru Holdings [Member]
Business Acquisition [Line Items]
Generating capacity | MW	29
Class B membership interest in Broadview Holdings [Member]
Business Acquisition [Line Items]
Ownership percentage		100.00%
Western Interconnect [Member]
Business Acquisition [Line Items]
Ownership percentage		99.00%
Initial distributable cash flow from Broadview [Member]
Business Acquisition [Line Items]
Ownership percentage		84.00%
Broadview [Member]
Business Acquisition [Line Items]
Generating capacity | MW		324
Western Interconnect [Member]
Business Acquisition [Line Items]
Transmission line length | mi		35
Transmission-level voltage | kV		345
Long-term debt, gross		$ 54,400