Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 276,200	$ 273,100
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	272,600	269,600
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	226,900	229,400
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	101,000	98,100
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	124,300	129,500
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,600	1,800
Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	44,100	37,700
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,600	2,500
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues	3,600	3,500
Regulated Electric | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	212,000	213,400
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	167,400	174,200
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	67,400	67,800
Regulated Electric | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	98,500	104,800
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,500	1,600
Regulated Electric | Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	44,100	37,700
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	500	1,500
Regulated Electric | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues	3,000	2,900
Regulated Natural Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	60,500	55,900
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	59,400	54,900
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	33,600	30,300
Regulated Natural Gas | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	25,800	24,600
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0	0
Regulated Natural Gas | Interchange [Member]
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	1,100	1,000
Regulated Natural Gas | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues	600	600
All Other | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	100	300
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	100	300
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0	0
All Other | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0	100
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	100	200
All Other | Interchange [Member]
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
All Other | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Total revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	276,200	273,100
Total revenues | Regulated Electric
Disaggregation of Revenue [Line Items]
Total operating revenues	215,000	216,300
Total revenues | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total operating revenues	61,100	56,500
Total revenues | All Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 100	$ 300