Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 538,563	$ 476,012
Regulatory Assets [Member] | Income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	277,434	230,434
Regulatory Assets [Member] | Pension
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	132,000	130,193
Regulatory Assets [Member] | Deferred Financing Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	31,089	34,080
Regulatory Assets [Member] | Employee related benefits
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	18,622	19,458
Regulatory Assets [Member] | Supply costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 35,454	10,532
Regulatory assets, remaining amortization period	1 year
Regulatory Assets [Member] | State And Local Taxes And Fees
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 7,146	15,532
Regulatory Assets [Member] | Environmental clean-up
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	11,179	11,221
Regulatory Assets [Member] | Other
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	25,639	24,562
Regulatory Liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	484,563	479,161
Regulatory Liabilities [Member] | Removal cost
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	442,129	428,528
Regulatory Liabilities [Member] | Tax Cuts and Jobs Act deferral
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 0	20,497
Regulatory liability, remaining amortization period	1 year
Regulatory Liabilities [Member] | Supply costs
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 14,226	15,453
Regulatory liability, remaining amortization period	1 year
Regulatory Liabilities [Member] | Gas storage sales
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 8,307	8,728
Regulatory liability, remaining amortization period	20 years
Regulatory Liabilities [Member] | Deferred Revenue
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 14,177	0
Regulatory liability, remaining amortization period	1 year
Regulatory Liabilities [Member] | State And Local Taxes And Fees
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 1,846	1,747
Regulatory liability, remaining amortization period	1 year
Regulatory Liabilities [Member] | Environmental clean-up
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 1,181	1,247
Regulatory Liabilities [Member] | Other
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 2,697	$ 2,961
Minimum [Member] | Regulatory Liabilities [Member] | Deferred Financing Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liability, remaining amortization period	1 year
Maximum [Member] | Regulatory Liabilities [Member] | Deferred Financing Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liability, remaining amortization period	13 years