Segments and Related Information (Details) (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
Segment Reporting Information [Line Items]
Intercompany revenue									$ 136,917		$ 125,344		$ 124,334
Operating revenues	240,388	231,060	210,175	241,415	217,395	226,475	194,173	223,799	923,038		861,842		875,790
Depreciation and amortization									76,897		69,234		68,574
Total interest charges and financing costs									25,816		22,937		23,993
Income Tax Expense (Benefit)									36,409		29,558		33,614
Net income	7,225	22,013	10,544	19,685	7,131	22,200	5,742	14,878	59,468		49,951		51,006
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									789,518		757,920		755,541
Depreciation and amortization									64,237		59,768		58,800
Total interest charges and financing costs									22,966		20,303		21,181
Income Tax Expense (Benefit)									33,691		27,164		32,656
Net income									51,334		45,377		47,093
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									134,834		103,827		121,028
Depreciation and amortization									12,485		9,251		9,599
Total interest charges and financing costs									2,749		2,554		2,675
Income Tax Expense (Benefit)									4,623		2,113		1,995
Net income									6,501		3,094		2,964
All Other
Segment Reporting Information [Line Items]
Operating revenues									1,003		1,177		1,207
Depreciation and amortization									175		215		175
Total interest charges and financing costs									101		80		137
Income Tax Expense (Benefit)									(1,905)		281		(1,037)
Net income									1,633		1,480		949
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues									923,038	[1]	861,842	[1]	875,790	[1]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									789,168	[1]	757,565	[1]	755,136	[1]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									132,867	[1]	103,100	[1]	119,447	[1]
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues									1,003	[1]	1,177	[1]	1,207	[1]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues									(2,317)		(1,082)		(1,986)
Depreciation and amortization									0		0		0
Total interest charges and financing costs									0		0		0
Income Tax Expense (Benefit)									0		0		0
Net income									0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									350		355		405
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									1,967		727		1,581
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues									$ 0		$ 0		$ 0

[1] Operating revenues include $137 million, $125 million and $124 million of intercompany revenue for the years ended Dec. 31, 2013, 2012 and 2011, respectively. See Note 15 for further discussion of related party transactions by operating segment.