Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 247,511		$ 246,144		$ 742,468		$ 720,167
Net income (loss)	22,325		24,221		58,985		54,477
Affiliate electric revenue	46,000		44,000		131,000		127,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	232,946		232,326		660,216		646,936
Net income (loss)	24,594		25,459		54,580		52,058
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	14,450		13,743		82,006		72,970
Net income (loss)	(2,184)		(1,201)		3,388		2,426
All Other
Segment Reporting Information [Line Items]
Operating revenues	315		282		847		877
Net income (loss)	(85)		(37)		1,017		(7)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	247,511	[1]	246,144	[1]	742,468	[2]	720,167	[2]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	232,802	[1]	232,216	[1]	659,853	[2]	646,619	[2]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	14,394		13,646		81,768		72,671
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	315		282		847		877
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(200)		(207)		(601)		(616)
Net income (loss)	0		0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	144		110		363		317
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	56		97		238		299
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

[1] (a) Operating revenues include $46 million and $44 million of affiliate electric revenue for the three months ended Sept. 30, 2017 and 2016, respectively.

[2] (a) Operating revenues include $131 million and $127 million of affiliate electric revenue for the nine months ended Sept. 30, 2017 and 2016, respectively.