Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 136,664	$ 125,084	$ 376,020	$ 310,244
Intersegment Revenues
Total operating revenues, unaffiliated customers		136,664	125,084	376,020	310,244
Operating Income
Total operating income		13,248	14,061	53,654	49,160
Other expense, net		(262)	(1,002)	(194)	(1,703)
Interest		3,881	3,073	7,545	5,811
Income Before Income Taxes		9,105	9,986	45,915	41,646
Income taxes		2,718	3,940	12,674	16,456
Net Income		6,387	6,046	33,241	25,190	$ 58,124
Identifiable Assets
Total identifiable assets		1,462,941		1,462,941		1,414,934
Regulated Energy [Member]
Operating Income
Total operating income		14,304	14,086	41,015	37,481
Identifiable Assets
Total identifiable assets		1,199,672		1,199,672		1,121,673
Unregulated Energy [Member]
Operating Income
Total operating income		490	2	14,174	11,577
Identifiable Assets
Total identifiable assets		227,191		227,191		259,041
Other [Member]
Identifiable Assets
Total identifiable assets		36,078		36,078		$ 34,220
Other and eliminations [Member]
Operating Income
Total operating income		(1,546)	(27)	(1,535)	102
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		136,664	125,084	376,020	310,244
Intersegment Revenues
Total operating revenues, unaffiliated customers		136,664	125,084	376,020	310,244
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		67,731	68,815	173,685	165,261
Intersegment Revenues
Total operating revenues, unaffiliated customers		67,731	68,815	173,685	165,261
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		68,933	56,269	202,335	144,983
Intersegment Revenues
Total operating revenues, unaffiliated customers		68,933	56,269	202,335	144,983
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	10,379	9,120	25,976	14,567
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	10,379	9,120	25,976	14,567
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	2,773	2,181	6,212	3,389
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	2,773	2,181	6,212	3,389
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	7,412	6,780	19,377	10,791
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	7,412	6,780	19,377	10,791
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	194	159	387	387
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 194	$ 159	$ 387	$ 387

[1] All significant intersegment revenues are billed at market rates and have been eliminated from consolidated operating revenues.