Fair Value Measurements - Fair Value Level 3 Inputs, Quantitative Information (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Energy derivative [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Energy derivative - fair value	$ 11,880	$ 26,872
Energy derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	2.60%	1.96%
Forward electricity price	32.70	71.45
Energy derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	2.40%	1.69%
Forward electricity price	20.65	14.44
Contingent Consideration Liability [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Contingent consideration - fair value	$ 124,868	$ 21,943	$ 128,000	$ 21,505	$ 21,502	$ 0
Contingent Consideration Liability [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Contingent consideration - fair value	24,771	$ 21,943
Contingent Consideration Liability [Member] | Discounted Cash Flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Annual energy production loss		1.00%
Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate		8.00%
Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate		4.00%
Energy derivative [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Energy derivative - fair value	$ 11,880	$ 26,872	$ 12,198	$ 30,782	$ 33,895	$ 40,916
Broadview Project [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Annual energy production loss	0.70%
Broadview Project [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	8.00%
Broadview Project [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	4.00%
Tsugaru Holdings [Member] | Contingent Consideration Liability [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Contingent consideration - fair value	$ 100,097
Tsugaru Holdings [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	6.90%
Tsugaru Holdings [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Deferred purchase price	$ 128,000
Tsugaru Holdings [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Deferred purchase price	$ 109,000