Fair Value Measurements - Schedule of Assets and Liabilities (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset	$ 31,308	$ 49,019
Derivative Liability	26,121	57,756
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset	29,134	41,012
Derivative Liability	21,850	52,784
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset	2,174	8,007
Derivative Liability	4,271	4,972
Electric Portfolio
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset	19,933	33,287
Derivative Liability	17,504	27,284
Electric Portfolio | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset	19,282	28,765
Derivative Liability	13,474	24,124
Electric Portfolio | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset	651	4,522
Derivative Liability	4,030	3,160
Natural Gas Portfolio
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset	11,375	15,732
Derivative Liability	8,617	30,472
Natural Gas Portfolio | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset	9,852	12,247
Derivative Liability	8,376	28,660
Natural Gas Portfolio | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset	1,523	3,485
Derivative Liability	$ 241	$ 1,812