Fair Value Measurements Valuation Technicques for Measurement with Unobservable Inputs (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) $ / MMBTU $ / MWh	Mar. 31, 2016 USD ($)	Dec. 31, 2035	Dec. 31, 2034	Dec. 31, 2033	Dec. 31, 2032	Dec. 31, 2031	Dec. 31, 2030	Dec. 31, 2029	Dec. 31, 2028	Dec. 31, 2027	Dec. 31, 2026	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2014 $ / MMBTU $ / MWh	Dec. 31, 2016 USD ($)	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value measurement, sensitivity analysis, hypothetical increase or decrease of market prices, result on fair value	10.00%																							10.00%
Fair Value Measurements, Sensitivity Analysis, Hypothetical Increase or Decrease of Market Prices, Result on Fair Value	$ 100,000																						$ 200,000
Finite-Lived Intangible Assets, Net	255,875,000
Impairment of Intangible Assets (Excluding Goodwill)	80,333,000
Carrying Value [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure	175,542,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Assets	29,047,000																						63,079,000
Derivative Liability	64,778,000																						60,571,000
Electric Portfolio [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	$ 1,800,000	$ 5,700,000
Electric Portfolio [Member] | Discounted cash flow [Member] | Low [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	9.00																					11.86
Electric Portfolio [Member] | Discounted cash flow [Member] | High [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	29.65																					33.52
Electric Portfolio [Member] | Discounted cash flow [Member] | Weighted Average [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	24.04																					27.61
Electric Portfolio [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Assets	$ 20,343,000																						36,460,000
Derivative Liability	$ 44,499,000																						41,329,000
Natural Gas Portfolio [Member] | Discounted cash flow [Member] | Low [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MMBtu) | $ / MMBTU	1.76																					2.00
Natural Gas Portfolio [Member] | Discounted cash flow [Member] | High [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MMBtu) | $ / MMBTU	3.11																					3.24
Natural Gas Portfolio [Member] | Discounted cash flow [Member] | Weighted Average [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MMBtu) | $ / MMBTU	2.18																					2.42
Natural Gas Portfolio [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Assets	$ 8,704,000																						26,619,000
Derivative Liability	20,279,000																						19,101,000
Commodity Contract [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Assets	29,047,000																						63,079,000
Derivative Liability	64,778,000																						$ 60,430,000
Wells Project [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-Lived Intangible Assets, Net	14,879,000
Impairment of Intangible Assets (Excluding Goodwill)	1,812,000
Wells Project [Member] | Carrying Value [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure	$ 13,067,000
Wells Project [Member] | Discounted cash flow [Member] | Low [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	8.76
Fair Value Inputs, Power Contract Costs	$ 3,965
Wells Project [Member] | Discounted cash flow [Member] | High [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	26.70
Fair Value Inputs, Power Contract Costs	$ 4,223
Wells Project [Member] | Discounted cash flow [Member] | Weighted Average [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	20.86
Fair Value Inputs, Power Contract Costs	$ 4,051
Rocky Reach Project [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-Lived Intangible Assets, Net	235,331,000
Impairment of Intangible Assets (Excluding Goodwill)	75,513,000
Rocky Reach Project [Member] | Carrying Value [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure	$ 159,818,000
Rocky Reach Project [Member] | Discounted cash flow [Member] | Low [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	8.53
Fair Value Inputs, Power Contract Costs	$ 5,827,000
Rocky Reach Project [Member] | Discounted cash flow [Member] | High [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	48.21
Fair Value Inputs, Power Contract Costs	$ 6,780,000
Rocky Reach Project [Member] | Discounted cash flow [Member] | Weighted Average [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	27.69
Fair Value Inputs, Power Contract Costs	$ 6,150,000
Priest Rapids Development [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-Lived Intangible Assets, Net	5,665,000
Impairment of Intangible Assets (Excluding Goodwill)	3,008,000
Priest Rapids Development [Member] | Carrying Value [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure	$ 2,657,000
Priest Rapids Development [Member] | Discounted cash flow [Member] | Low [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	13.70
Fair Value Inputs, Power Contract Costs	$ 620
Priest Rapids Development [Member] | Discounted cash flow [Member] | High [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	29.38
Fair Value Inputs, Power Contract Costs	$ 4,022
Priest Rapids Development [Member] | Discounted cash flow [Member] | Weighted Average [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price (per MWh) | $ / MWh	23.14
Fair Value Inputs, Power Contract Costs	$ 2,306
Scenario, Forecast [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Comparability Adjustments			24.40%	24.40%	24.40%	24.40%	24.40%	24.40%	24.40%	24.40%	24.40%	24.40%	24.40%	24.40%	24.40%	14.10%	14.10%	14.10%	14.10%	14.10%	14.10%