Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Operating revenues	$ 929,590	$ 869,501	$ 1,591,520	$ 1,610,031
Regulatory cost recovery revenue	15,000	12,000	29,000	32,000
Electric Service | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	515,128	432,568	840,201	784,134
Electric Service | Non-Residential
Disaggregation of Revenue [Line Items]
Operating revenues	381,121	395,929	684,472	728,597
Electric Service | Wholesale energy sales
Disaggregation of Revenue [Line Items]
Operating revenues	15,927	21,991	30,595	58,443
Transmission services for others
Disaggregation of Revenue [Line Items]
Operating revenues	14,766	15,157	30,693	30,406
Other sources
Disaggregation of Revenue [Line Items]
Operating revenues	2,648	3,856	5,559	8,451
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 915,000	$ 858,000	$ 1,563,000	$ 1,578,000