Fair Value Measurements - Fair Value Level 3 Inputs, Quantitative Information (Details) - USD ($)
$ in Thousands |
6 Months Ended |
12 Months Ended |
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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] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Energy derivative - fair value |
$ 12,198
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$ 26,872
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| Energy derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Discount rate |
2.53%
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1.96%
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| Forward electricity price |
127.76
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71.45
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| Energy derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Discount rate |
2.34%
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1.69%
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| Forward electricity price |
28.51
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14.44
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| Contingent Consideration Liability [Member] | Fair Value, Measurements, Recurring [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Contingent consideration - fair value |
$ 128,000
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$ 21,943
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$ 130,901
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$ 21,502
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$ 0
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$ 0
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| Contingent Consideration Liability [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Contingent consideration - fair value |
24,497
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$ 21,943
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| Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Annual energy production loss |
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1.00%
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| Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Maximum [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Discount rate |
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8.00%
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| Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Minimum [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Discount rate |
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4.00%
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| Energy derivative [Member] | Fair Value, Measurements, Recurring [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Energy derivative - fair value |
$ 12,198
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$ 26,872
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$ 15,824
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$ 33,895
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$ 38,559
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$ 40,916
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| Broadview Project [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Annual energy production loss |
0.70%
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| Broadview Project [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Maximum [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Discount rate |
8.00%
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| Broadview Project [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Minimum [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Discount rate |
4.00%
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| Tsugaru Holdings [Member] | Contingent Consideration Liability [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Contingent consideration - fair value |
$ 100,650
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| Tsugaru Holdings [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Discount rate |
6.90%
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| Tsugaru Holdings [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Maximum [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Deferred purchase price |
$ 128,000
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| Tsugaru Holdings [Member] | Contingent Consideration Liability [Member] | Discounted Cash Flow [Member] | Minimum [Member] |
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
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| Deferred purchase price |
$ 109,000
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