Revenue (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 692,714
Regulatory cost recovery revenue	10,000
Electric Service | Retail Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	316,675
Electric Service | Retail Non-Residential
Disaggregation of Revenue [Line Items]
Total operating revenues	343,189
Electric Service | Wholesale Energy Sales
Disaggregation of Revenue [Line Items]
Total operating revenues	12,089
Transmission Services For Others
Disaggregation of Revenue [Line Items]
Total operating revenues	14,845
Other Sources
Disaggregation of Revenue [Line Items]
Total operating revenues	5,916
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 683,000