Fair Value - Schedule of Activity For Energy Commodity Derivative Assets (Liabilities) Measured At Fair Value Using Significant Unobservable Inputs (Level 3) (Details) - Level 3 [Member] - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ (2,976)	$ (5,262)	$ (16,409)
Included in regulatory assets	(4,311)	8,610	5,353
Settlements	(1,123)	(6,324)	5,794
Ending Balance	(8,410)	(2,976)	(5,262)
Natural Gas Exchange Agreements [Member]
Fair Value Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(2,976)	(2,774)	(3,164)
Included in regulatory assets	(4,311)	8,175	326
Settlements	(1,123)	(8,377)	64
Ending Balance	$ (8,410)	(2,976)	(2,774)
Power Exchange Agreements [Member]
Fair Value Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		(2,488)	(13,245)
Included in regulatory assets		435	5,027
Settlements		$ 2,053	5,730
Ending Balance			$ (2,488)

Fair Value - Schedule of Activity For Energy Commodity Derivative Assets (Liabilities) Measured At Fair Value Using Significant Unobservable Inputs (Level 3) (Details) - Level 3 [Member] - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2020

Dec. 31, 2019

Dec. 31, 2018

Fair Value Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]

Beginning Balance

$ (2,976)

$ (5,262)

$ (16,409)

Included in regulatory assets

(4,311)

8,610