Revenue - Schedule of Gas Operating Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 504,793	$ 552,180	$ 1,337,907	$ 1,379,596
Alternative revenue program revenues (deferrals)	(3,765)	(34,545)	5,668	(15,775)
Other revenues	1,799	3,042	7,514	20,271
Total Gas operating revenues	502,827	520,677	1,351,089	1,384,092
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	378,555	417,228	934,115	959,837
Small commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	82,463	89,610	241,970	256,750
Large commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	12,667	13,962	47,640	51,714
Industrial/other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6,702	6,478	22,298	23,457
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 24,406	$ 24,902	$ 91,884	$ 87,838