Revenue - Schedule of Gas Operating Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 241,300	$ 203,082	$ 1,068,164	$ 966,941	$ 1,440,374	$ 1,346,391
Alternative revenue program revenues (deferrals)	12,569	9,199	(5,335)	9,545	(2,740)	7,629
Other revenues	1,979	(1,447)	7,747	(391)	7,432	1,646
Total Gas operating revenues	255,848	210,834	1,070,576	976,095	1,445,066	1,355,666
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	147,326	131,008	743,791	690,861	1,011,450	957,379
Small commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	48,283	35,204	185,774	159,122	248,193	228,720
Large commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,199	9,942	40,030	32,588	52,075	45,493
Industrial/other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9,608	5,888	30,352	19,089	37,505	25,435
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 21,884	$ 21,040	$ 68,217	$ 65,281	$ 91,151	$ 89,364