Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 185,160	$ 146,296
Intersegment Revenues
Total operating revenues, unaffiliated customers		185,160	146,296
Operating Income
Total operating income		34,676	36,380
Other expense, net		(277)	(34)
Interest		2,739	2,650
Income Before Income Taxes		31,660	33,696
Income taxes		12,516	13,329
Net Income		19,144	20,367	$ 44,675
Identifiable Assets
Total identifiable assets		1,238,311		1,229,219
Regulated Energy [Member]
Operating Income
Total operating income		23,017	24,319
Identifiable Assets
Total identifiable assets		1,000,265		986,752
Unregulated Energy [Member]
Operating Income
Total operating income		11,530	11,936
Identifiable Assets
Total identifiable assets		213,078		226,368
Other [Member]
Identifiable Assets
Total identifiable assets		24,968		$ 16,099
Other and eliminations [Member]
Operating Income
Total operating income		129	125
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		185,160	146,296
Intersegment Revenues
Total operating revenues, unaffiliated customers		185,160	146,296
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		96,446	88,894
Intersegment Revenues
Total operating revenues, unaffiliated customers		96,446	88,894
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		88,714	57,402
Intersegment Revenues
Total operating revenues, unaffiliated customers		88,714	57,402
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	5,447	661
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	5,447	661
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	1,208	322
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	1,208	322
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	4,011	113
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	4,011	113
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	228	226
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 228	$ 226

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