Utility Plant (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Public Utility, Property, Plant and Equipment
Accumulated amortization of capital leases	$ 1,300	$ 700
Utility Plant
Distribution plant	6,122,739	5,670,351
Production plant	3,099,805	3,068,135
Transmission plant	1,442,854	1,361,495
General plant	682,976	586,226
Intangible plant (including capitalized software)	662,328	447,568
Plant acquisition adjustment	242,826	242,826
Underground storage	35,404	31,815
Liquefied natural gas storage	12,628	12,628
Plant held for future use	39,384	53,428
Recoverable cushion gas	8,655	8,655
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment	239,857	275,014
Capital leases, net of accumulated amortization	1,315	1,129
Less: Accumulated depreciation and amortization	(2,832,321)	(2,428,524)
Subtotal	9,758,450	9,330,746
Construction work in progress	550,466	495,937
Net utility plant	$ 10,308,916	9,826,683
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
Underground storage, Estimated Useful Life	25 years
Liquefied natural gas storage, Estimated Useful Life	25 years
Public Utilities, Property, Plant and Equipment, Plant Not Classified, Estimated Useful Life	1 year
Capital leases, net of accumulated amortization, Estimated Useful Life	4 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
Underground storage, Estimated Useful Life	60 years
Liquefied natural gas storage, Estimated Useful Life	60 years
Public Utilities, Property, Plant and Equipment, Plant Not Classified, Estimated Useful Life	125 years
Capital leases, net of accumulated amortization, Estimated Useful Life	6 years
Subsidiaries [Member]
Utility Plant
Distribution plant	$ 7,722,024	7,289,998
Production plant	3,974,250	3,954,057
Transmission plant	1,550,950	1,471,337
General plant	718,105	628,179
Intangible plant (including capitalized software)	652,942	438,185
Plant acquisition adjustment	282,792	282,792
Underground storage	48,874	45,288
Liquefied natural gas storage	14,498	14,498
Plant held for future use	39,536	53,580
Recoverable cushion gas	8,655	8,655
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment	239,857	275,014
Capital leases, net of accumulated amortization	1,315	1,129
Less: Accumulated depreciation and amortization	(5,495,348)	(5,131,966)
Subtotal	9,758,450	9,330,746
Construction work in progress	550,466	495,937
Net utility plant	$ 10,308,916	$ 9,826,683