Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 125,084	$ 102,342	$ 310,244	$ 248,638
Intersegment Revenues
Total operating revenues, unaffiliated customers		125,084	102,342	310,244	248,638
Operating Income
Total operating income		13,666	15,742	48,341	52,122
Other expense, net		(607)	(8)	(884)	(42)
Interest		3,073	2,624	5,811	5,274
Income Before Income Taxes		9,986	13,110	41,646	46,806
Income taxes		3,940	5,081	16,456	18,410
Net Income		6,046	8,029	25,190	28,396	$ 44,675
Identifiable Assets
Total identifiable assets		1,256,988		1,256,988		1,229,219
Regulated Energy [Member]
Operating Income
Total operating income		13,730	15,226	36,747	39,545
Identifiable Assets
Total identifiable assets		1,025,745		1,025,745		986,752
Unregulated Energy [Member]
Operating Income
Total operating income		(38)	412	11,492	12,347
Identifiable Assets
Total identifiable assets		202,730		202,730		226,368
Other [Member]
Identifiable Assets
Total identifiable assets		28,513		28,513		$ 16,099
Other and eliminations [Member]
Operating Income
Total operating income		(26)	104	102	230
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		125,084	102,342	310,244	248,637
Intersegment Revenues
Total operating revenues, unaffiliated customers		125,084	102,342	310,244	248,637
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		68,815	66,590	165,261	155,483
Intersegment Revenues
Total operating revenues, unaffiliated customers		68,815	66,590	165,261	155,483
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		56,269	35,752	144,983	93,154
Intersegment Revenues
Total operating revenues, unaffiliated customers		56,269	35,752	144,983	93,154
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	9,120	2,097	14,567	2,759
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	9,120	2,097	14,567	2,759
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	2,181	805	3,389	1,128
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	2,181	805	3,389	1,128
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	6,780	1,052	10,791	1,165
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	6,780	1,052	10,791	1,165
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	159	240	387	466
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 159	$ 240	$ 387	$ 466

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