Revenue - Schedule of Gas Operating Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 259,066	$ 253,408	$ 763,859	$ 805,588	$ 1,343,565	$ 1,371,305
Alternative revenue program revenues (deferrals)	4,111	3,392	346	(31,153)	6,387	(22,776)
Other revenues (1)	(743)	1,911	1,056	4,953	4,860	18,595
Total Gas operating revenues	262,434	258,711	765,261	779,388	1,354,812	1,367,124
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	181,298	164,873	559,853	582,101	950,540	958,008
Small commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	41,455	50,184	123,918	139,794	233,241	251,281
Large commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9,979	11,123	22,646	25,085	46,496	49,703
Industrial/other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6,499	5,413	13,201	11,891	23,384	23,379
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 19,835	$ 21,815	$ 44,241	$ 46,717	$ 89,904	$ 88,934