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Fair Value Measurements - Recurring (Significant Unobservable Inputs in Level 3 Valuation) (Details) - Significant unobservable inputs (Level 3)
12 Months Ended
Dec. 31, 2018
USD ($)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]  
Term of counterparty option to renew gas purchase agreement 5 years
Commodity contracts | Minimum  
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]  
Fair value inputs commodity price $ 0.58
Commodity contracts | Maximum  
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]  
Fair value inputs commodity price $ 1.01
Embedded derivatives in commodity contracts  
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]  
Probability of renewal, first term 90.00%
Probability of renewal, second term 80.00%