Segment Information (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 231,046,000		$ 231,060,000		$ 744,302,000		$ 682,650,000
Net income (loss)	20,030,000		22,013,000		56,287,000		52,242,000
Affiliate electric revenue	33,000,000		36,000,000		97,000,000		101,000,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	216,382,000		216,634,000		625,971,000		592,980,000
Net income (loss)	21,227,000		22,859,000		48,092,000		46,530,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	14,909,000		15,228,000		122,005,000		90,727,000
Net income (loss)	(2,003,000)		(1,726,000)		5,565,000		4,100,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	273,000		249,000		825,000		742,000
Net income (loss)	806,000		880,000		2,630,000		1,612,000
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	231,046,000	[1]	231,060,000	[1]	744,302,000	[2]	682,650,000	[2]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	216,289,000	[1]	216,545,000	[1]	625,663,000	[2]	592,730,000	[2]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	14,484,000		14,266,000		117,814,000		89,178,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	273,000		249,000		825,000		742,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(518,000)		(1,051,000)		(4,499,000)		(1,799,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	93,000		89,000		308,000		250,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	425,000		962,000		4,191,000		1,549,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

[1] Operating revenues include $33 million and $36 million of affiliate electric revenue for the three months ended Sept. 30, 2014 and 2013, respectively.

[2] Operating revenues include $97 million and $101 million of affiliate electric revenue for the nine months ended Sept. 30, 2014 and 2013, respectively.