Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 239,356	$ 185,160
Intersegment Revenues
Total operating revenues, unaffiliated customers		239,356	185,160
Operating Income
Total operating income		40,406	35,099
Other expense, net		68	(700)
Interest		3,664	2,739
Income Before Income Taxes		36,810	31,660
Income taxes		9,955	12,516
Net Income		26,855	19,144	$ 58,124
Identifiable Assets
Total identifiable assets		1,427,450		1,417,434
Regulated Energy [Member]
Operating Income
Total operating income		26,711	23,395
Identifiable Assets
Total identifiable assets		1,148,635		1,121,673
Unregulated Energy [Member]
Operating Income
Total operating income		13,684	11,575
Identifiable Assets
Total identifiable assets		246,230		261,541
Other [Member]
Identifiable Assets
Total identifiable assets		32,585		$ 34,220
Other and eliminations [Member]
Operating Income
Total operating income		11	129
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		239,356	185,160
Intersegment Revenues
Total operating revenues, unaffiliated customers		239,356	185,160
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		105,954	96,446
Intersegment Revenues
Total operating revenues, unaffiliated customers		105,954	96,446
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		133,402	88,714
Intersegment Revenues
Total operating revenues, unaffiliated customers		133,402	88,714
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	15,598	5,447
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	15,598	5,447
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	3,439	1,208
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	3,439	1,208
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	11,965	4,011
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	11,965	4,011
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	194	228
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 194	$ 228

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