Segment Information - Schedule of Segment Reporting Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	$ 109,516	$ 78,175	$ 83,897	$ 120,914	$ 103,988	$ 80,610	$ 86,831	$ 146,597	$ 392,502	$ 418,027	$ 427,546
Intersegment Revenues
Total intersegment revenues									5,112	2,161	2,323
Operating Income
Operating Income	18,543	7,564	10,455	20,073	15,495	5,594	7,776	24,839	56,635	53,705	51,930
Other income									271	906	195
Interest charges									8,747	9,000	9,146
Income Before Income taxes									48,159	45,611	42,979
Income taxes									19,296	17,989	16,923
Net Income	9,857	3,219	5,060	10,727	7,957	2,397	3,520	13,747	28,863	27,622	26,056
Depreciation and Amortization
Total depreciation and amortization									22,510	20,153	18,536
Capital Expenditures
Total capital expenditures									78,210	44,431	46,955
Identifiable Assets
Total identifiable assets	733,746				709,066				733,746	709,066
Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers									245,042	255,405	268,830
Intersegment Revenues
Total intersegment revenues									1,166	1,368	1,104
Operating Income
Operating Income									46,999	43,911	43,267
Depreciation and Amortization
Total depreciation and amortization									18,653	16,512	14,680
Capital Expenditures
Total capital expenditures									69,056	37,104	41,898
Identifiable Assets
Total identifiable assets	615,438				565,563				615,438	565,563
Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers									130,020	149,586	146,430
Intersegment Revenues
Total intersegment revenues									3,029		363
Operating Income
Operating Income									8,355	9,619	8,150
Depreciation and Amortization
Total depreciation and amortization									3,420	3,229	3,569
Capital Expenditures
Total capital expenditures									3,969	2,432	2,764
Identifiable Assets
Total identifiable assets	79,287				107,916				79,287	107,916
Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers									17,440	13,036	12,286
Intersegment Revenues
Total intersegment revenues									917	793	856
Operating Income
Operating Income									1,281	175	513
Capital Expenditures
Total capital expenditures									5,185	4,895	2,293
Identifiable Assets
Total identifiable assets	39,021				35,587				39,021	35,587
Other and eliminations [Member]
Depreciation and Amortization
Total depreciation and amortization									$ 437	$ 412	$ 287