Revenues (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		$ 228,272	$ 226,578	$ 497,837	$ 487,937
Alternative revenue and other		3,482	3,448	7,061	7,020
Total operating revenues		231,754	230,026	$ 504,898	494,957
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs				24 months
Tolerance band				2
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	[1]	231,754	230,026	$ 504,898	494,957
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		187,473	182,180	416,853	399,284
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		69,656	63,769	167,801	156,460
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		115,799	116,503	245,191	239,165
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		2,018	1,908	3,861	3,659
Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		38,370	42,965	76,044	85,343
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		2,429	1,433	4,940	3,310
Regulated Electric
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		206,613	207,819	420,017	421,109
Alternative revenue and other		2,889	2,923	5,851	5,942
Total operating revenues		209,608	210,865	426,083	427,270
Regulated Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	[1]	209,502	210,742	425,868	427,051
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		166,883	164,331	341,109	334,274
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		58,734	54,754	126,537	120,797
Regulated Electric | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		106,367	107,899	211,184	210,279
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,782	1,678	3,388	3,198
Regulated Electric | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		38,370	42,965	76,044	85,343
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,360	523	2,864	1,492
Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		21,388	18,502	77,271	66,296
Alternative revenue and other		593	525	1,210	1,078
Total operating revenues		22,108	19,095	78,756	67,556
Regulated Natural Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues		21,981	19,027	78,481	67,374
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		20,319	17,592	75,195	64,478
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		10,912	9,015	41,238	35,643
Regulated Natural Gas | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		9,407	8,577	33,957	28,835
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
Regulated Natural Gas | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,069	910	2,076	1,818
All Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		271	257	549	532
Alternative revenue and other		0	0	0	0
Total operating revenues		271	257	549	532
All Other | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues		271	257	549	532
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		271	257	549	532
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		10	0	26	20
All Other | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		25	27	50	51
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		236	230	473	461
All Other | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
All Other | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		$ 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.