Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2016		Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 219,173		$ 216,813		$ 474,023		$ 490,773
Net income (loss)	12,625		12,512		30,256		34,779
Affiliate electric revenue	43,000		33,000		84,000		78,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	202,977		199,719		414,610		410,112
Net income (loss)	15,098		13,787		26,599		29,301	[1]
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	16,127		16,962		59,227		80,483
Net income (loss)	(2,465)		(1,074)		3,627		5,707
All Other
Segment Reporting Information [Line Items]
Operating revenues	289		303		595		683
Net income (loss)	(8)		(201)		30		(229)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	219,173	[2]	216,813	[2]	474,023	[3]	490,773	[3]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	202,879	[2]	199,621	[2]	414,403	[3]	409,905	[3]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	16,005		16,889		59,025		80,185
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	289		303		595		683
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(220)		(171)		(409)		(505)
Net income (loss)	0		0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	98		98		207		207
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	122		73		202		298
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 $43 million and $33 million of affiliate electric revenue for the three months ended June 30, 2016 and 2015, respectively.

[3] (a) Operating revenues include $84 million and $78 million of affiliate electric revenue for the six months ended June 30, 2016 and 2015, respectively.