Revenue - Schedule of Gas Operating Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 203,082	$ 200,256	$ 966,941	$ 1,005,844	$ 1,346,391	$ 1,374,724
Alternative revenue program revenues (deferrals)	9,199	7,957	9,545	(23,196)	7,629	(23,913)
Other revenues (1)	(1,447)	1,767	(391)	6,720	1,646	8,770
Total Gas operating revenues	210,834	209,980	976,095	989,368	1,355,666	1,359,581
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	131,008	124,169	690,861	706,270	957,379	961,928
Small commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	35,204	39,725	159,122	179,519	228,720	250,986
Large commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9,942	10,945	32,588	36,030	45,493	49,288
Industrial/other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5,888	3,837	19,089	15,728	25,435	21,826
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 21,040	$ 21,580	$ 65,281	$ 68,297	$ 89,364	$ 90,696