Acquisitions - Additional Information (Detail) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) Dekatherm	Dec. 31, 2017 USD ($)
Cash paid for acquisition									$ 23,988	$ 16,654	$ 11,945
Revenues	$ 131,973	$ 92,626	$ 94,541	$ 160,464	$ 134,214	$ 93,400	$ 93,872	$ 168,831	479,604	490,316	449,646
Operating Income (Loss)	29,642	14,358	18,164	44,123	28,873	12,879	12,238	40,853	106,287	94,843	89,730
Net Income	22,562	$ 5,622	$ 8,304	$ 28,665	17,801	$ 5,538	$ 6,385	$ 26,856	65,153	$ 56,580	58,124
Marlin Gas Services [Member]
Revenues									5,702
Operating Income (Loss)									1,500
Additional Compressed Natural Gas Deliverability | Dekatherm										7,000
Boulden [Member]
Revenues									550
Operating Income (Loss)									239
Business Combination, Consideration Transferred									24,600
Marlin and Ohl [Member]
Business Combination, Consideration Transferred										$ 18,200
ARM Energy [Member]
Business Combination, Liabilities Arising from Contingencies, Amount Recognized									2,500
Ohl [Member]
Public Utilities, Property, Plant and Equipment, Amount of Acquisition Adjustments					$ 200					200
Regulated Energy [Member]
Operating Income (Loss)									$ 86,584	79,215	74,584
Regulated Energy [Member] | Elkton Gas [Member]
Number of customers acquired through acquisition									7,000
Unregulated Energy
Operating Income (Loss)									$ 19,939	17,124	$ 14,941
Unregulated Energy | Boulden [Member]
Number of customers acquired through acquisition									5,200
Business Combination, Working Capital	200								$ 200
Business Combination, Contingent Consideration, Liability	600								600
Property, Plant and Equipment, Acquired During Period	8,300								8,300
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	$ 5,100								5,100
Goodwill, Acquired During Period									11,200
Unregulated Energy | Marlin [Member]
Goodwill, Acquired During Period										$ 4,800
Unregulated Energy | Ohl [Member]
Revenues									1,662
Operating Income (Loss)									$ 385
Number of customers acquired through acquisition										2,500
Goodwill, Acquired During Period										$ 1,500