Segment Information - Additional Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 90,317	$ 91,239	$ 499,108	$ 479,441
Cost of gas	22,603	25,538	163,167	175,697
Environmental remediation	967	1,022	7,258	7,528
Operating Segments [Member] \| Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Natural Gas Distribution revenues	77,916	84,965	472,691	461,306
Other regulated services	5,147	112	7,397	219
Operating revenues	83,063	85,077	480,088	461,525
Cost of gas	22,659	25,593	163,335	175,864
Environmental remediation	967	1,022	7,258	7,528
Revenue taxes	3,534	3,522	19,956	20,731
NGD margin	$ 55,903	$ 54,940	$ 289,539	$ 257,402

The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities.

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