Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Operating revenues		$ 231,754	$ 230,026	$ 504,898	$ 494,957
Net income (loss)		15,189	14,241	46,605	36,660
Affiliate electric revenue		38,000	43,000	76,000	85,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		209,608	210,865	426,083	427,270
Net income (loss)		15,481	14,516	35,925	29,986
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		22,108	19,095	78,756	67,556
Net income (loss)		(148)	(973)	10,250	5,572
All Other
Segment Reporting Information [Line Items]
Operating revenues		271	257	549	532
Net income (loss)		(144)	698	430	1,102
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1]	231,754	230,026	504,898	494,957
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]	209,502	210,742	425,868	427,051
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		21,981	19,027	78,481	67,374
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues		271	257	549	532
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(233)	(191)	(490)	(401)
Net income (loss)		0	0	0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		106	123	215	219
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		127	68	275	182
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues		$ 0	$ 0	$ 0	$ 0

[1] (a) Operating revenues include $76 million and $85 million of affiliate electric revenue for the six months ended June 30, 2018 and 2017, respectively.