Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ (7,034)	$ (27,674)	$ 22,773
Regulatory Assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	852,579	806,227
Regulatory Assets [Member] | Flow-through income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	509,038	464,663
Regulatory Assets [Member] | Supply costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 101,096	88,329
Regulatory assets, remaining amortization period	18 months
Regulatory Assets [Member] | Pension
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 87,965	98,336
Regulatory Assets [Member] | Excess deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	54,364	60,813
Regulatory Assets [Member] | Employee related benefits
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	27,920	21,648
Regulatory Assets [Member] | Deferred financing costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	22,620	25,636
Regulatory Assets [Member] | State & local taxes & fees
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	15,684	6,520
Regulatory Assets [Member] | Environmental clean-up
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	10,963	11,262
Regulatory Assets [Member] | Other
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	22,929	29,020
Regulatory Liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	675,358	666,939
Regulatory Liabilities [Member] | Removal cost
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	502,289	479,294
Regulatory Liabilities [Member] | Excess deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	148,989	158,047
Regulatory Liabilities [Member] | Supply costs
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 11,536	16,430
Regulatory liability, remaining amortization period	1 year
Regulatory Liabilities [Member] | Gas storage sales
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 7,046	7,466
Regulatory liability, remaining amortization period	18 years
Regulatory Liabilities [Member] | State & local taxes & fees
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 2,327	3,021
Regulatory liability, remaining amortization period	1 year
Regulatory Liabilities [Member] | Environmental clean-up
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 592	508
Regulatory Liabilities [Member] | Rates subject to refund
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 0	1,971
Regulatory liability, remaining amortization period	1 year
Regulatory Liabilities [Member] | Other
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 2,579	$ 202
Minimum [Member] | Regulatory Assets [Member] | Deferred financing costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, remaining amortization period	1 year
Maximum [Member] | Regulatory Assets [Member] | Deferred financing costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, remaining amortization period	13 years