Utility Plant (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Public Utility, Property, Plant and Equipment
Accumulated amortization of capital leases	$ 1,000	$ 1,300
Utility Plant
Distribution plant	6,602,934	6,122,739
Production plant	3,066,792	3,099,805
Transmission plant	1,463,288	1,442,854
General plant	698,275	682,976
Intangible plant (including capitalized software)	735,826	662,328
Plant acquisition adjustment	242,826	242,826
Underground storage	37,511	35,404
Liquefied natural gas storage	12,628	12,628
Plant held for future use	46,233	39,384
Base Gas Stored Underground	8,655	8,655
Plant not classified	316,923	239,857
Capital leases, net of accumulated amortization	1,488	1,315
Less: Accumulated depreciation and amortization	(3,236,240)	(2,832,321)
Subtotal	9,997,139	9,758,450
Construction work in progress	591,199	550,466
Net utility plant	$ 10,588,338	10,308,916
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	5 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	$ 8,185,700	7,722,024
Production plant	3,743,493	3,974,250
Transmission plant	1,571,186	1,550,950
General plant	731,279	718,105
Intangible plant (including capitalized software)	726,383	652,942
Plant acquisition adjustment	282,792	282,792
Underground storage	50,963	48,874
Liquefied natural gas storage	14,498	14,498
Plant held for future use	46,385	39,536
Base Gas Stored Underground	8,655	8,655
Plant not classified	316,923	239,857
Capital leases, net of accumulated amortization	1,488	1,315
Less: Accumulated depreciation and amortization	(5,682,606)	(5,495,348)
Subtotal	9,997,139	9,758,450
Construction work in progress	591,199	550,466
Net utility plant	$ 10,588,338	$ 10,308,916