Revenues (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		$ 269,565	$ 261,359
Alternative revenue and other		3,579	3,572
Total operating revenues		$ 273,144	264,931
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]	$ 273,144	264,931
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		229,380	217,104
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		98,145	92,691
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		129,392	122,662
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,843	1,751
Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		37,674	42,378
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		2,511	1,877
Regulated Electric
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		213,404	213,290
Alternative revenue and other		2,962	3,019
Total operating revenues		216,475	216,405
Regulated Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	[1]	216,366	216,309
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		174,226	169,943
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		67,803	66,043
Regulated Electric | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		104,817	102,380
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,606	1,520
Regulated Electric | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		37,674	42,378
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,504	969
Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		55,883	47,794
Alternative revenue and other		617	553
Total operating revenues		56,648	48,461
Regulated Natural Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues		56,500	48,347
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		54,876	46,886
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		30,326	26,628
Regulated Natural Gas | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		24,550	20,258
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0
Regulated Natural Gas | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,007	908
All Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		278	275
Alternative revenue and other		0	0
Total operating revenues		278	275
All Other | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues		278	275
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		278	275
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		16	20
All Other | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		25	24
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		237	231
All Other | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0
All Other | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		$ 0	$ 0

[1] (a) Operating revenues include $38 million and $42 million of affiliate electric revenue for the three months ended March 31, 2018 and 2017, respectively.