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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 21,900	$ 21,650
Regulatory assets, long-term	168,395	144,138
Regulatory liabilities, current	16,663	7,480
Regulatory liabilities, long -term	233,973	239,906
Deferred Income Taxes
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	0	0
Regulatory liabilities, long -term	134,719	141,707
Plant Removal Obligations
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	0	0
Regulatory liabilities, long -term	98,707	97,726
Fuel Clause Adjustment
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	10,947	3,982
Regulatory liabilities, long -term	$ 0	0
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)	1 year
Alternative Revenue Program Riders
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	$ 3,581	2,857
Regulatory liabilities, long -term	$ 470	0
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)	1 year
Pension and Other Postretirement Benefit Plans
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	$ 1,959	471
Regulatory liabilities, long -term	$ 0	0
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)	1 year
Deferred Rate Case Expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	$ 0	0
Regulatory liabilities, long -term	0	401
ISO Cost Recovery Trackers
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	0	0
Regulatory liabilities, long -term	$ 10	0
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)	2 years
Other
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	$ 176	170
Regulatory liabilities, long -term	67	72
Pension and Other Postretirement Benefit Plans
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	11,037	9,090
Regulatory assets, long-term	146,071	130,783
Alternative Revenue Program Riders
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	8,871	8,464
Regulatory assets, long-term	$ 9,373	2,844
Regulatory Assets - Remaining Recovery/Refund Period (Month)	3 years
Asset Retirement Obligations1
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 0	0
Regulatory assets, long-term	8,462	7,772
ISO Cost Recovery Trackers
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	1,079	2,033
Regulatory assets, long-term	$ 867	1,170
Regulatory Assets - Remaining Recovery/Refund Period (Month)	2 years
Unrecovered Project Costs1
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 361	859
Regulatory assets, long-term	$ 2,989	478
Regulatory Assets - Remaining Recovery/Refund Period (Month)	3 years
Deferred Rate Case Expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 360	260
Regulatory assets, long-term	230	489
Debt Reacquisition Premiums1
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	192	201
Regulatory assets, long-term	$ 341	548
Regulatory Assets - Remaining Recovery/Refund Period (Month)	30 years
Other1
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 0	743
Regulatory assets, long-term	$ 62	$ 54

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Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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