Segment Information - Schedule of Segment Reporting Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	$ 120,380	$ 91,619	$ 100,497	$ 186,337	$ 122,887	$ 86,545	$ 94,146	$ 140,729	$ 498,834	$ 444,306	$ 392,502
Intersegment Revenues (1)
Total intersegment revenues	120,380	91,619	100,497	186,337	122,887	86,545	94,146	140,729	498,834	444,306	392,502
Operating Income
Operating Income	12,408	7,792	10,457	31,623	18,312	8,720	9,152	26,550	62,279	62,734	56,635
Gain (Loss) on Disposition of Business									7,139
Other income									101	372	271
Interest charges									9,482	8,234	8,747
Income Before Income taxes									60,037	54,872	48,159
Income taxes									23,945	22,085	19,296
Net Income	10,097	3,180	5,134	17,681	9,683	3,879	4,356	14,869	36,092	32,787	28,863
Depreciation and Amortization
Total depreciation and amortization									26,316	23,965	22,510
Capital Expenditures
Total capital expenditures									98,057	108,039	78,210
Identifiable Assets
Total identifiable assets	904,469				837,522				904,469	837,522
Regulated Energy
Operating Income
Operating Income									50,451	50,084	46,999
Depreciation and Amortization
Total depreciation and amortization									21,915	19,822	18,653
Capital Expenditures
Total capital expenditures									84,959	95,944	69,056
Identifiable Assets
Total identifiable assets	796,021				708,950				796,021	708,950
Unregulated Energy
Operating Income
Operating Income									11,723	12,353	8,355
Depreciation and Amortization
Total depreciation and amortization									3,994	3,686	3,420
Capital Expenditures
Total capital expenditures									9,648	4,829	3,969
Identifiable Assets
Total identifiable assets	84,732				100,585				84,732	100,585
Other
Operating Income
Operating Income									105	297	1,281
Capital Expenditures
Total capital expenditures									3,450	7,266	5,185
Identifiable Assets
Total identifiable assets	23,716				27,987				23,716	27,987
Other and eliminations
Depreciation and Amortization
Total depreciation and amortization									407	457	437
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									498,834	444,306	392,502
Intersegment Revenues (1)
Total intersegment revenues									498,834	444,306	392,502
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									299,345	263,573	245,042
Intersegment Revenues (1)
Total intersegment revenues									299,345	263,573	245,042
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									184,557	161,760	130,020
Intersegment Revenues (1)
Total intersegment revenues									184,557	161,760	130,020
Operating Segments | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									14,932	18,973	17,440
Intersegment Revenues (1)
Total intersegment revenues									14,932	18,973	17,440
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									2,480	7,044	5,112
Intersegment Revenues (1)
Total intersegment revenues									2,480	7,044	5,112
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									1,097	1,064	1,166
Intersegment Revenues (1)
Total intersegment revenues									1,097	1,064	1,166
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									404	4,963	3,029
Intersegment Revenues (1)
Total intersegment revenues									404	4,963	3,029
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues									979	1,017	917
Intersegment Revenues (1)
Total intersegment revenues									$ 979	$ 1,017	$ 917