Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Operating revenues		$ 255,997	$ 247,511	$ 760,895	$ 742,468
Net income (loss)		31,008	22,325	77,613	58,985
Affiliate electric revenue		40,000	46,000	116,000	131,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		240,456	232,946	666,539	660,216
Net income (loss)		34,256	24,594	70,181	54,580
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		15,322	14,450	94,078	82,006
Net income (loss)		(3,448)	(2,184)	6,802	3,388
All Other
Segment Reporting Information [Line Items]
Operating revenues		298	315	847	847
Net income (loss)		200	(85)	630	1,017
Total revenues
Segment Reporting Information [Line Items]
Operating revenues	[1]	255,997	247,511	760,895	742,468
Total revenues | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]	240,344	232,802	666,212	659,853
Total revenues | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		15,355	14,394	93,836	81,768
Total revenues | All Other
Segment Reporting Information [Line Items]
Operating revenues		298	315	847	847
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(79)	(200)	(569)	(601)
Net income (loss)		0	0	0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		112	144	327	363
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		(33)	56	242	238
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues		$ 0	$ 0	$ 0	$ 0

[1] (a) Operating revenues include $116 million and $131 million of affiliate electric revenue for the nine months ended Sept. 30, 2018 and 2017, respectively.