Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Operating revenues	$ 661,930	$ 740,530
Regulatory cost recovery revenue	14,000	20,000
Electric Service | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	325,073	351,566
Electric Service | Non-Residential
Disaggregation of Revenue [Line Items]
Operating revenues	303,351	332,668
Electric Service | Wholesale energy sales
Disaggregation of Revenue [Line Items]
Operating revenues	14,668	36,452
Transmission services for others
Disaggregation of Revenue [Line Items]
Operating revenues	15,927	15,249
Other sources
Disaggregation of Revenue [Line Items]
Operating revenues	2,911	4,595
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 648,000	$ 721,000