Revenue - Schedule of Gas Operating Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 261,699	$ 248,242	$ 740,257	$ 713,882	$ 1,289,256	$ 1,242,203
Alternative revenue program revenues (deferrals)	10,393	10,135	37,602	5,903	67,046	17,280
Other revenues	3,587	1,785	(7,867)	2,979	(6,766)	3,945
Total Gas operating revenues	275,679	260,162	769,992	722,764	1,349,536	1,263,428
Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues	166,702	158,442	511,313	501,179	867,338	850,344
Small Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues	55,653	51,018	143,596	132,143	254,966	235,684
Large Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues	13,134	12,781	28,574	26,376	54,577	49,561
Industrial/other [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,491	5,043	12,001	10,494	23,533	20,157
Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 20,719	$ 20,958	$ 44,773	$ 43,690	$ 88,842	$ 86,457