Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2015		Sep. 30, 2014		Sep. 30, 2015		Sep. 30, 2014
Segment Reporting Information [Line Items]
Operating revenues	$ 236,161		$ 231,046		$ 726,934		$ 744,302
Net income (loss)	26,232		20,030		61,011		56,287
Affiliate electric revenue	42,000		33,000		121,000		97,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	224,767		216,382		634,879		625,971
Net income (loss)	28,285		21,227		57,586	[1]	48,092
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	11,265		14,909		91,748		122,005
Net income (loss)	(1,993)		(2,003)		3,714		5,565
All Other
Segment Reporting Information [Line Items]
Operating revenues	407		273		1,090		825
Net income (loss)	(60)		806		(289)		2,630
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	236,161	[2]	231,046	[2]	726,934	[3]	744,302	[3]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	224,666	[2]	216,289	[2]	634,571	[3]	625,663	[3]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	11,088		14,484		91,273		117,814
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	407		273		1,090		825
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(278)		(518)		(783)		(4,499)
Net income (loss)	0		0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	101		93		308		308
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	$ 177		425		475		4,191
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues			$ 0		$ 0		$ 0

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

[2] Operating revenues include $42 million and $33 million of affiliate electric revenue for the three months ended Sept. 30, 2015 and 2014

[3] Operating revenues include $121 million and $97 million of affiliate electric revenue for the nine months ended Sept. 30, 2015 and 2014, respectively.