Segment Information - Schedule of Segment Reporting Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	$ 122,887	[1]	$ 86,545	[1]	$ 94,146	[1]	$ 140,729	[1]	$ 109,516	[1]	$ 78,175	[1]	$ 83,897	[1]	$ 120,914	[1]	$ 444,306		$ 392,502		$ 418,027
Intersegment Revenues (1)
Total intersegment revenues	122,887	[1]	86,545	[1]	94,146	[1]	140,729	[1]	109,516	[1]	78,175	[1]	83,897	[1]	120,914	[1]	444,306		392,502		418,027
Operating Income
Operating Income	18,312	[1]	8,720	[1]	9,152	[1]	26,550	[1]	18,543	[1]	7,564	[1]	10,455	[1]	20,073	[1]	62,734		56,635		53,705
Other income																	372		271		906
Interest charges																	8,234		8,747		9,000
Income Before Income taxes																	54,872		48,159		45,611
Income taxes																	22,085		19,296		17,989
Net Income	9,683	[1]	3,879	[1]	4,356	[1]	14,869	[1]	9,857	[1]	3,219	[1]	5,060	[1]	10,727	[1]	32,787		28,863		27,622
Depreciation and Amortization
Total depreciation and amortization																	23,965		22,510		20,153
Capital Expenditures
Total capital expenditures																	108,039		78,210		44,431
Identifiable Assets
Total identifiable assets	837,522								733,746								837,522		733,746
Regulated Energy
Operating Income
Operating Income																	50,084		46,999		43,911
Depreciation and Amortization
Total depreciation and amortization																	19,822		18,653		16,512
Capital Expenditures
Total capital expenditures																	95,944		69,056		37,104
Identifiable Assets
Total identifiable assets	708,950								615,438								708,950		615,438
Unregulated Energy
Operating Income
Operating Income																	12,353		8,355		9,619
Depreciation and Amortization
Total depreciation and amortization																	3,686		3,420		3,229
Capital Expenditures
Total capital expenditures																	4,829		3,969		2,432
Identifiable Assets
Total identifiable assets	100,585								79,287								100,585		79,287
Other
Operating Income
Operating Income																	297		1,281		175
Capital Expenditures
Total capital expenditures																	7,266		5,185		4,895
Identifiable Assets
Total identifiable assets	27,987								39,021								27,987		39,021
Other and eliminations
Depreciation and Amortization
Total depreciation and amortization																	457		437		412
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues																	444,306		392,502		418,027
Intersegment Revenues (1)
Total intersegment revenues																	444,306		392,502		418,027
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues																	263,573		245,042		255,405
Intersegment Revenues (1)
Total intersegment revenues																	263,573		245,042		255,405
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues																	161,760		130,020		149,586
Intersegment Revenues (1)
Total intersegment revenues																	161,760		130,020		149,586
Operating Segments | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues																	18,973		17,440		13,036
Intersegment Revenues (1)
Total intersegment revenues																	18,973		17,440		13,036
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues																	7,044	[2]	5,112	[2]	1,614	[2]
Intersegment Revenues (1)
Total intersegment revenues																	7,044	[2]	5,112	[2]	1,614	[2]
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues																	1,064	[2]	1,166	[2]	821	[2]
Intersegment Revenues (1)
Total intersegment revenues																	1,064	[2]	1,166	[2]	821	[2]
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues																	4,963	[2]	3,029	[2]	0	[2]
Intersegment Revenues (1)
Total intersegment revenues																	4,963	[2]	3,029	[2]	0	[2]
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues																	1,017	[2]	917	[2]	793	[2]
Intersegment Revenues (1)
Total intersegment revenues																	$ 1,017	[2]	$ 917	[2]	$ 793	[2]

[1] The sum of the four quarters does not equal the total year due to rounding.

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