Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended		12 Months Ended
		Jun. 30, 2015		Jun. 30, 2014		Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 92,682		$ 100,497		$ 262,763	$ 286,834
Intersegment Revenues
Total operating revenues, unaffiliated customers		92,682		100,497		262,763	286,834
Operating Income
Total operating income		13,170		10,457		50,678	42,079
Other income, net of other expenses		(171)		405		(38)	413
Interest		2,485		2,303		4,933	4,459
Income Before Income Taxes		10,514		8,559		45,707	38,033
Income taxes		4,220		3,425		18,304	15,218
Net Income		6,294		5,134		27,403	22,815	$ 36,092
Identifiable Assets
Total identifiable assets		970,636				970,636		904,469
Regulated Energy [Member]
Operating Income
Total operating income		13,605		10,711		35,788	31,802
Identifiable Assets
Total identifiable assets		800,719				800,719		796,021
Unregulated Energy [Member]
Operating Income
Total operating income		(540)		(43)		14,689	10,815
Identifiable Assets
Total identifiable assets		147,102				147,102		84,732
Other [Member]
Identifiable Assets
Total identifiable assets		22,815				22,815		$ 23,716
Other and eliminations [Member]
Operating Income
Total operating income		105		(211)		201	(538)
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		92,682		100,497		262,763	286,834
Intersegment Revenues
Total operating revenues, unaffiliated customers		92,682		100,497		262,763	286,834
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		61,790		61,348		171,082	163,222
Intersegment Revenues
Total operating revenues, unaffiliated customers		61,790		61,348		171,082	163,222
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		30,892		34,299		91,681	114,173
Intersegment Revenues
Total operating revenues, unaffiliated customers		30,892		34,299		91,681	114,173
Operating Revenues, Unaffiliated Customers [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		0		4,850		0	9,439
Intersegment Revenues
Total operating revenues, unaffiliated customers		0		4,850		0	9,439
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		2,156	[1]	568	[1]	2,873	1,213
Intersegment Revenues
Total operating revenues, unaffiliated customers		2,156	[1]	568	[1]	2,873	1,213
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	270		298		560	590
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	270		298		560	590
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	1,666		22		1,873	121
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	1,666		22		1,873	121
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	220		248		440	502
Intersegment Revenues
Total operating revenues, unaffiliated customers	[1]	$ 220		$ 248		$ 440	$ 502

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