Revenues (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total operating revenues		$ 255,997	$ 247,511	$ 760,895	$ 742,468
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
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		252,613	244,117	$ 750,450	732,054
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		211,317	196,033	628,170	595,317
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		74,952	68,501	242,753	224,961
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		134,778	125,959	379,969	365,124
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,587	1,573	5,448	5,232
Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		39,993	45,916	116,037	131,259
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,303	2,168	6,243	5,478
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues		3,384	3,394	10,445	10,414
Regulated Electric
Disaggregation of Revenue [Line Items]
Total operating revenues		240,456	232,946	666,539	660,216
Regulated Electric | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		237,536	229,922	657,553	651,031
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		197,180	182,849	538,289	517,123
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		67,919	62,025	194,456	182,822
Regulated Electric | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		127,936	119,495	339,120	329,774
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,325	1,329	4,713	4,527
Regulated Electric | Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		39,993	45,916	116,037	131,259
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		363	1,157	3,227	2,649
Regulated Electric | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues		2,808	2,880	8,659	8,822
Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total operating revenues		15,322	14,450	94,078	82,006
Regulated Natural Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		14,779	13,880	92,050	80,176
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		13,839	12,869	89,034	77,347
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		7,021	6,428	48,259	42,071
Regulated Natural Gas | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		6,818	6,441	40,775	35,276
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
Regulated Natural Gas | Interchange [Member]
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		940	1,011	3,016	2,829
Regulated Natural Gas | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues		576	514	1,786	1,592
All Other
Disaggregation of Revenue [Line Items]
Total operating revenues		298	315	847	847
All Other | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		298	315	847	847
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		298	315	847	847
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		12	48	38	68
All Other | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		24	23	74	74
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		262	244	735	705
All Other | Interchange [Member]
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
All Other | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues		0	0	0	0
Total revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	[1]	255,997	247,511	760,895	742,468
Total revenues | Regulated Electric
Disaggregation of Revenue [Line Items]
Total operating revenues	[1]	240,344	232,802	666,212	659,853
Total revenues | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total operating revenues		15,355	14,394	93,836	81,768
Total revenues | All Other
Disaggregation of Revenue [Line Items]
Total operating revenues		$ 298	$ 315	$ 847	$ 847

[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.