Acquisitions - Additional Information (Detail) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) Dekatherm	Dec. 31, 2018 USD ($)
Cash paid for acquisition									$ 22,231	$ 23,988	$ 16,654
Revenues	$ 137,038	$ 101,419	$ 97,051	$ 152,690	$ 131,974	$ 92,626	$ 94,542	$ 160,464	488,198	479,605	490,316
Operating Income (Loss)	35,206	17,406	17,977	42,134	29,641	14,357	18,165	44,122	112,723	106,285	94,844
Net Income	22,352	$ 9,261	$ 10,956	$ 28,930	$ 22,564	$ 5,621	$ 8,304	$ 28,663	71,498	$ 65,153	56,580
Discontinued Operations [Member]
Business Combination, Working Capital	8,000								8,000
Elkton Gas [Member]
Revenues									2,399
Operating Income (Loss)									418
Marlin Gas Services [Member]
Additional Compressed Natural Gas Deliverability | Dekatherm										7,000
Boulden [Member]
Revenues									5,717	$ 550
Operating Income (Loss)									1,854	239
Business Combination, Consideration Transferred									24,600
Western Natural Gas [Member]
Revenues									555
Operating Income (Loss)									90
Regulated Energy [Member]
Operating Income (Loss)									$ 92,124	86,584	79,215
Regulated Energy [Member] | Elkton Gas [Member]
Number of customers acquired through acquisition									7,000
Business Combination, Consideration Transferred									$ 15,600
Business Combination, Working Capital	600								600
Property, Plant and Equipment, Acquired During Period	15,900								15,900
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	2,600								2,600
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Regulatory Liabilities									2,600
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net	4,300								4,300
Unregulated Energy
Operating Income (Loss)									$ 20,664	$ 19,938	$ 17,125
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 | Western Natural Gas [Member]
Business Combination, Consideration Transferred									6,700
Business Combination, Working Capital	300								300
Business Combination, Contingent Consideration, Liability	300								300
Property, Plant and Equipment, Acquired During Period	3,500								3,500
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net	1,800								1,800
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets	$ 1,400								$ 1,400