Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 246,144		$ 236,161		$ 720,167		$ 726,934
Net income (loss)	24,221		26,232		54,477		61,011
Affiliate electric revenue	44,000		42,000		127,000		121,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	232,326		224,767		646,936		634,879
Net income (loss)	25,459		28,285		52,058		57,586	[1]
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	13,743		11,265		72,970		91,748
Net income (loss)	(1,201)		(1,993)		2,426		3,714
All Other
Segment Reporting Information [Line Items]
Operating revenues	282		407		877		1,090
Net income (loss)	(37)		(60)		(7)		(289)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	246,144	[2]	236,161	[2]	720,167	[3]	726,934	[3]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	232,216	[2]	224,666	[2]	646,619	[3]	634,571	[3]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	13,646		11,088		72,671		91,273
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	282		407		877		1,090
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(207)		(278)		(616)		(783)
Net income (loss)	0		0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	110		101		317		308
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	97		177		299		475
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

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

[2] (a) Operating revenues include $44 million and $42 million of affiliate electric revenue for the three months ended Sept. 30, 2016 and 2015, respectively.

[3] (a) Operating revenues include $127 million and $121 million of affiliate electric revenue for the nine months ended Sept. 30, 2016 and 2015, respectively.