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	$ 474,687	$ 472,587	$ 916,580	$ 910,021
Alternative revenue and other	6,651	7,209	11,990	29,847
Total operating revenues	481,338	479,796	$ 928,570	939,868
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	297,230	312,235	$ 582,714	602,405
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	85,107	84,188	165,156	163,789
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	200,760	215,805	396,531	416,762
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	11,363	12,242	21,027	21,854
Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	115,629	101,893	208,861	193,034
Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	58,970	56,394	114,616	110,572
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 2,858	$ 2,065	$ 10,389	$ 4,010