Fair Value Measurements - Valuation Techniques (Details)	3 Months Ended			9 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) $ / MWh	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) $ / MWh	Sep. 30, 2017 USD ($) $ / MWh	Dec. 31, 2018 USD ($) $ / MWh $ / MMBTU	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of Intangible Assets (Excluding Goodwill)					$ 1,907,000	$ 81,345,000
Fair Value measurement, sensitivity analysis, hypothetical increase or decrease of market prices, result on fair value, percent					10.00%
Fair Value Measurements, Sensitivity Analysis, Hypothetical Increase or Decrease of Market Prices, Result on Fair Value					$ 2,600,000
Energy derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net of Amounts Presented in the Statement of Financial Position					49,019,000	24,405,000
Net of Amounts Presented in the Statement of Financial Position					$ 57,756,000	$ 86,094,000
Natural Gas Portfolio | Income Approach Valuation Technique | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Price Per Millions of BTU | $ / MMBTU					1.84
Natural Gas Portfolio | Income Approach Valuation Technique | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Price Per Millions of BTU | $ / MMBTU					5.80
Natural Gas Portfolio | Income Approach Valuation Technique | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Price Per Millions of BTU | $ / MMBTU					3.18
Natural Gas Portfolio | Parent Company [Member] | Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net of Amounts Presented in the Statement of Financial Position					$ 3,485,000
Net of Amounts Presented in the Statement of Financial Position					$ 1,812,000
Electric Portfolio | Income Approach Valuation Technique | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh					11.35
Electric Portfolio | Income Approach Valuation Technique | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh					66.45
Electric Portfolio | Income Approach Valuation Technique | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh					29.63
Electric Portfolio | Parent Company [Member] | Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net of Amounts Presented in the Statement of Financial Position					$ 4,522,000
Net of Amounts Presented in the Statement of Financial Position					$ 3,160,000
Wells Project1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finite-Lived Intangible Assets, Net	$ 4,302,000	$ 10,621,000	$ 14,879,000	$ 10,621,000
Impairment of Intangible Assets (Excluding Goodwill)	$ 1,907,000	1,012,000	$ 1,812,000
Wells Project1 | Income Approach Valuation Technique | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh	9.69		8.76	14.06
Fair Value Inputs, Power Contract Costs	$ 4,126		$ 3,965	$ 4,126
Wells Project1 | Income Approach Valuation Technique | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh	25.30		26.70	26.86
Fair Value Inputs, Power Contract Costs	$ 4,126		$ 4,223	$ 4,126
Wells Project1 | Income Approach Valuation Technique | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh	17.50		20.86	22.24
Fair Value Inputs, Power Contract Costs	$ 4,126		$ 4,051	$ 4,126
Rocky Reach Project
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finite-Lived Intangible Assets, Net			235,331,000
Impairment of Intangible Assets (Excluding Goodwill)			75,513,000
Rocky Reach Project | Income Approach Valuation Technique | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh	8.53
Fair Value Inputs, Power Contract Costs	$ 5,827
Rocky Reach Project | Income Approach Valuation Technique | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh	48.21
Fair Value Inputs, Power Contract Costs	$ 6,780
Rocky Reach Project | Income Approach Valuation Technique | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh	27.69
Fair Value Inputs, Power Contract Costs	$ 6,150
Priest Rapids Development
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finite-Lived Intangible Assets, Net			5,665,000
Impairment of Intangible Assets (Excluding Goodwill)			3,008,000
Priest Rapids Development | Income Approach Valuation Technique | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh	13.70
Fair Value Inputs, Power Contract Costs	$ 620
Priest Rapids Development | Income Approach Valuation Technique | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh	29.38
Fair Value Inputs, Power Contract Costs	$ 4,022
Priest Rapids Development | Income Approach Valuation Technique | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Power prices (per MWh) | $ / MWh	23.14
Fair Value Inputs, Power Contract Costs	$ 2,306
Carrying Amount | Wells Project1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure	$ 2,395,000	$ 9,609,000	13,067,000	$ 9,609,000
Carrying Amount | Rocky Reach Project
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure			159,818,000
Carrying Amount | Priest Rapids Development
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure			$ 2,657,000