Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 230,026		$ 219,173		$ 494,957		$ 474,023
Net income (loss)	14,241		12,625		36,660		30,256
Affiliate electric revenue	43,000		43,000		85,000		84,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	210,865		202,977		427,270		414,610
Net income (loss)	14,516		15,098		29,986		26,599
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	19,095		16,127		67,556		59,227
Net income (loss)	(973)		(2,465)		5,572		3,627
All Other
Segment Reporting Information [Line Items]
Operating revenues	257		289		532		595
Net income (loss)	698		(8)		1,102		30
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	230,026	[1]	219,173	[1]	494,957	[2]	474,023	[2]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	210,742	[1]	202,879	[1]	427,051	[2]	414,403	[2]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	19,027		16,005		67,374		59,025
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	257		289		532		595
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(191)		(220)		(401)		(409)
Net income (loss)	0		0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	123		98		219		207
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	68		122		182		202
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

[1] (a) Operating revenues include $43 million of affiliate electric revenue for the three months ended June 30, 2017 and 2016.

[2] (a) Operating revenues include $85 million and $84 million of affiliate electric revenue for the six months ended June 30, 2017 and 2016, respectively.