Segment Information - Additional Segment Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 773,679	$ 746,372	$ 706,143
Cost of gas	262,755	254,911	255,519
Environmental remediation	9,691	12,337	11,127
Revenue taxes	30,291	30,325	30,082
Operating Segments [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
NGD operating revenues	721,950	708,472	680,386
Other regulated services	19,122	12,056	262
Operating revenues	741,072	720,528	680,648
Cost of gas	262,980	255,135	255,743
Environmental remediation	9,691	12,337	11,127
Revenue taxes	30,291	30,325	30,082
NGD margin	$ 438,110	$ 422,731	$ 383,696