Fair Value Measurements - Fair Value Level 3 Inputs, Quantitative Information (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 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	$ 12,198	$ 26,872
Energy derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	2.53%	1.96%
Forward electricity price	127.76	71.45
Energy derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	2.34%	1.69%
Forward electricity price	28.51	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	$ 128,000	$ 21,943	$ 130,901	$ 21,502	$ 0	$ 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	24,497	$ 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	$ 12,198	$ 26,872	$ 15,824	$ 33,895	$ 38,559	$ 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	$ 100,650
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