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	$ 441,893	$ 437,434
Alternative revenue and other	5,339	22,638
Total operating revenues	$ 447,232	460,072
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs	24 months
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 285,484	290,170
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	80,049	79,601
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	195,771	200,957
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	9,664	9,612
Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	93,232	91,141
Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	55,646	54,178
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 7,531	$ 1,945