Segments and Related Information (Details) (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
Segment Reporting Information [Line Items]
Intercompany revenue									$ 145,102		$ 136,917		$ 125,344
Operating revenues	256,160	231,046	228,114	285,142	240,388	231,060	210,175	241,415	1,000,462		923,038		861,842
Depreciation and amortization									79,654		76,897		69,234
Total interest charges and financing costs									25,913		25,816		22,937
Income tax expense (benefit)									42,403		36,409		29,558
Net income (loss)	14,355	20,030	12,022	24,235	7,225	22,013	10,544	19,685	70,642		59,468		49,951
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									830,245		789,518		757,920
Depreciation and amortization									65,978		64,237		59,768
Total interest charges and financing costs									23,448		22,966		20,303
Income tax expense (benefit)									39,621		33,691		27,164
Net income (loss)									59,060		51,334		45,377
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									174,514		134,834		103,827
Depreciation and amortization									13,501		12,485		9,251
Total interest charges and financing costs									2,358		2,749		2,554
Income tax expense (benefit)									5,993		4,623		2,113
Net income (loss)									8,714		6,501		3,094
All Other
Segment Reporting Information [Line Items]
Operating revenues									1,085		1,003		1,177
Depreciation and amortization									175		175		215
Total interest charges and financing costs									107		101		80
Income tax expense (benefit)									(3,211)		(1,905)		281
Net income (loss)									2,868		1,633		1,480
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues									1,000,462	[1]	923,038	[1]	861,842	[1]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									829,748	[1]	789,168	[1]	757,565	[1]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									169,629	[1]	132,867	[1]	103,100	[1]
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues									1,085	[1]	1,003	[1]	1,177	[1]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues									(5,382)		(2,317)		(1,082)
Depreciation and amortization									0		0		0
Total interest charges and financing costs									0		0		0
Income tax expense (benefit)									0		0		0
Net income (loss)									0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									497		350		355
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									4,885		1,967		727
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues									$ 0		$ 0		$ 0

[1] Operating revenues include $145 million, $137 million and $125 million of intercompany revenue for the years ended Dec. 31, 2014, 2013 and 2012 respectively. See Note 15 for further discussion of related party transactions by operating segment.