Segment Information (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 228,114,000		$ 210,175,000		$ 513,256,000		$ 451,590,000
Net income (loss)	12,022,000		10,544,000		36,257,000		30,229,000
Affiliate electric revenue	33,000,000		33,000,000		63,000,000		65,000,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	198,400,000		185,457,000		409,589,000		376,346,000
Net income (loss)	11,799,000		10,068,000		26,865,000		23,671,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	30,388,000		24,833,000		107,096,000		75,498,000
Net income (loss)	(141,000)		201,000		7,568,000		5,825,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	298,000		246,000		552,000		493,000
Net income (loss)	364,000		275,000		1,824,000		733,000
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	228,114,000	[1]	210,175,000	[1]	513,256,000	[2]	451,590,000	[2]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	198,277,000	[1]	185,374,000	[1]	409,374,000	[2]	376,185,000	[2]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	29,539,000		24,555,000		103,330,000		74,912,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	298,000		246,000		552,000		493,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(972,000)		(361,000)		(3,981,000)		(747,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	123,000		83,000		215,000		161,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	849,000		278,000		3,766,000		586,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

[1] Operating revenues include $33 million of affiliate electric revenue for the three months ended June 30, 2014 and 2013.

[2] Operating revenues include $63 million and $65 million of affiliate electric revenue for the six months ended June 30, 2014 and 2013, respectively.