Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,190,787	$ 1,268,034	$ 2,800,818	$ 2,934,871
Regulatory cost recovery revenue	13,000	11,000	45,000	38,000
Electric Service | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	668,467	695,480	1,452,601	1,512,402
Electric Service | Non-Residential
Disaggregation of Revenue [Line Items]
Operating revenues	465,602	496,809	1,194,199	1,275,498
Electric Service | Wholesale energy sales
Disaggregation of Revenue [Line Items]
Operating revenues	36,775	53,501	95,218	80,982
Transmission services for others
Disaggregation of Revenue [Line Items]
Operating revenues	15,841	15,902	46,247	46,235
Other sources
Disaggregation of Revenue [Line Items]
Operating revenues	4,102	6,342	12,553	19,754
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,178,000	$ 1,257,000	$ 2,756,000	$ 2,897,000