Revenue - Schedule of Gas Operating Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 200,256	$ 196,837	$ 1,005,844	$ 937,094	$ 1,374,724	$ 1,283,934
Alternative revenue program revenues (deferrals)	7,957	9,094	(23,196)	46,696	(23,913)	67,017
Other revenues	1,767	11,592	6,720	3,725	8,770	3,049
Total Gas operating revenues	209,980	217,523	989,368	987,515	1,359,581	1,354,000
Tax Reform Savings Revenue Adjustments
Disaggregation of Revenue [Line Items]
Other revenues	(800)	(1,000)	(3,700)	(13,500)	(3,700)	(13,500)
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	124,169	120,249	706,270	631,562	961,928	864,128
Small commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	39,725	40,020	179,519	183,616	250,986	253,330
Large commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10,945	11,360	36,030	39,934	49,288	54,462
Industrial/other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3,837	5,390	15,728	17,391	21,826	23,899
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 21,580	$ 19,818	$ 68,297	$ 64,591	$ 90,696	$ 88,115