Utility Plant (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Public Utility, Property, Plant and Equipment
Accumulated amortization of capital leases	$ 1,600	$ 1,000
Utility Plant
Distribution plant	7,028,731	6,602,934
Production plant	3,096,092	3,066,792
Transmission plant	1,494,781	1,463,288
General plant	697,501	698,275
Intangible plant (including capitalized software)1	779,767	735,826
Plant acquisition adjustment	242,826	242,826
Underground storage	39,498	37,511
Liquefied natural gas storage	12,628	12,628
Plant held for future use	45,929	46,233
Recoverable Cushion Gas	8,655	8,655
Plant not classified	384,794	316,923
Finance leases, net of accumulated amortization2	881	1,488
Less: accumulated provision for depreciation	(3,671,094)	(3,236,240)
Subtotal	10,160,989	9,997,139
Construction work in progress	712,204	591,199
Net utility plant	$ 10,873,193	10,588,338
Minimum
Public Utility, Property, Plant and Equipment
Distribution plant, Estimated Useful Life	20 years
Production plant, Estimated Useful Life	12 years
Transmission plant, Estimated Useful Life	43 years
General plant, Estimated Useful Life	5 years
Public Utilities, Property, Plant and Equipment, Intangible, Estimated Useful Life 1	3 years
Underground storage, Estimated Useful Life	25 years
Liquefied natural gas storage, Estimated Useful Life	25 years
Minimum | Franchise Rights [Member]
Public Utility, Property, Plant and Equipment
General plant, Estimated Useful Life	10 years
Minimum | Software and Software Development Costs [Member]
Public Utility, Property, Plant and Equipment
General plant, Estimated Useful Life	3 years
Maximum
Public Utility, Property, Plant and Equipment
Distribution plant, Estimated Useful Life	65 years
Production plant, Estimated Useful Life	90 years
Transmission plant, Estimated Useful Life	75 years
General plant, Estimated Useful Life	75 years
Public Utilities, Property, Plant and Equipment, Intangible, Estimated Useful Life 1	50 years
Underground storage, Estimated Useful Life	60 years
Liquefied natural gas storage, Estimated Useful Life	60 years
Maximum | Franchise Rights [Member]
Public Utility, Property, Plant and Equipment
General plant, Estimated Useful Life	50 years
Maximum | Software and Software Development Costs [Member]
Public Utility, Property, Plant and Equipment
General plant, Estimated Useful Life	10 years
Subsidiaries [Member]
Utility Plant
Distribution plant	$ 8,592,720	8,185,700
Production plant	3,767,014	3,743,493
Transmission plant	1,601,731	1,571,186
General plant	726,327	731,279
Intangible plant (including capitalized software)1	770,317	726,383
Plant acquisition adjustment	282,792	282,792
Underground storage	52,927	50,963
Liquefied natural gas storage	14,498	14,498
Plant held for future use	46,081	46,385
Recoverable Cushion Gas	8,655	8,655
Plant not classified	384,794	316,923
Finance leases, net of accumulated amortization2	881	1,488
Less: accumulated provision for depreciation	(6,087,748)	(5,682,606)
Subtotal	10,160,989	9,997,139
Construction work in progress	712,204	591,199
Net utility plant	$ 10,873,193	$ 10,588,338