Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2015		Jun. 30, 2014		Jun. 30, 2015		Jun. 30, 2014
Segment Reporting Information [Line Items]
Operating revenues	$ 216,813		$ 228,114		$ 490,773		$ 513,256
Net income (loss)	12,512		12,022		34,779		36,257
Affiliate electric revenue	33,000		0		78,000		63,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	199,719		198,400		410,112		409,589
Net income (loss)	13,787		11,799		29,301	[1]	26,865
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	16,962		30,388		80,483		107,096
Net income (loss)	(1,074)		(141)		5,707		7,568
All Other
Segment Reporting Information [Line Items]
Operating revenues	303		298		683		552
Net income (loss)	(201)		364		(229)		1,824
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	216,813	[2]	228,114	[2]	490,773	[3]	513,256	[3]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	199,621	[2]	198,277	[2]	409,905	[3]	409,374	[3]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	16,889		29,539		80,185		103,330
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	303		298		683		552
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(171)		(972)		(505)		(3,981)
Net income (loss)	0		0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	98		123		207		215
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	73		849		298		3,766
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

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

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

[3] Operating revenues include $78 million and $63 million of affiliate electric revenue for the six months ended June 30, 2015 and 2014, respectively.