Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Segment Reporting Information [Line Items]
Operating revenues	$ 273,960,000		$ 285,142,000
Net income (loss)	22,267,000		24,235,000
Affiliate electric revenue	38,000,000		31,000,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	210,393,000		211,189,000
Net income (loss)	15,514,000	[1]	15,066,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	63,521,000		76,708,000
Net income (loss)	6,781,000		7,709,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	380,000		254,000
Net income (loss)	(28,000)		1,460,000
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	273,960,000	[2]	285,142,000	[2]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	210,284,000	[2]	211,097,000	[2]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	63,296,000		73,791,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	380,000		254,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(334,000)		(3,009,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	109,000		92,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	225,000		2,917,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0

[1] Includes a net of tax charge related to the Monticello LCM/EPU project. See Note 5.

[2] Operating revenues include $38 million and $31 million of affiliate electric revenue for the three months ended March 31, 2015 and 2014, respectively.