Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Regulatory Assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 391,985	$ 476,012
Regulatory Assets [Member] | Income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	163,605	230,434
Regulatory Assets [Member] | Pension
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	115,504	130,193
Regulatory Assets [Member] | Deferred Financing Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	37,090	34,080
Regulatory Assets [Member] | Employee related benefits
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	17,729	19,458
Regulatory Assets [Member] | Supply costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 13,398	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	$ 10,896	15,532
Regulatory Assets [Member] | Environmental clean-up
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	12,399	11,221
Regulatory Assets [Member] | Other
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	21,364	24,562
Regulatory Liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 433,043	479,161
Regulatory Liabilities [Member] | Deferred Financing Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liability, remaining amortization period	1 year
Regulatory Liabilities [Member] | Removal cost
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 408,451	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	$ 10,357	15,453
Regulatory liability, remaining amortization period	1 year
Regulatory Liabilities [Member] | Gas storage sales
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 9,149	8,728
Regulatory liability, remaining amortization period	21 years
Regulatory Liabilities [Member] | Deferred Revenue
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 2,201	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	$ 1,520	1,747
Regulatory liability, remaining amortization period	1 year
Regulatory Liabilities [Member] | Environmental clean-up
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 1,365	1,247
Regulatory Liabilities [Member] | Other
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 0	$ 2,961