Fair Value Measurements - Unobservable Input (Details) - Wells Project [Member] - Income Approach Valuation Technique	3 Months Ended
	Mar. 31, 2018 USD ($) $ / MWh
Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Power prices (per MWh) | $ / MWh	9.69
Fair Value Inputs, Power Contract Costs | $	$ 4,126
Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Power prices (per MWh) | $ / MWh	25.30
Fair Value Inputs, Power Contract Costs | $	$ 4,126
Weighted Average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Power prices (per MWh) | $ / MWh	17.50
Fair Value Inputs, Power Contract Costs | $	$ 4,126