Revenue (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,268,034	$ 2,934,871
Regulatory cost recovery revenue	11,000	38,000
Electric Service | Retail residential electric service
Disaggregation of Revenue [Line Items]
Operating revenues	695,480	1,512,402
Electric Service | Retail non-residential electric service
Disaggregation of Revenue [Line Items]
Operating revenues	496,809	1,275,498
Electric Service | Wholesale energy sales
Disaggregation of Revenue [Line Items]
Operating revenues	53,501	80,982
Transmission Services For Others
Disaggregation of Revenue [Line Items]
Operating revenues	15,902	46,235
Other sources
Disaggregation of Revenue [Line Items]
Operating revenues	6,342	19,754
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,257,000	$ 2,897,000