Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Operating revenues	$ 740,530	$ 692,714
Regulatory cost recovery revenue	20,000	10,000
Electric Service | Retail residential electric service
Disaggregation of Revenue [Line Items]
Operating revenues	351,566	316,675
Electric Service | Retail non-residential electric service
Disaggregation of Revenue [Line Items]
Operating revenues	332,668	343,189
Electric Service | Wholesale energy sales
Disaggregation of Revenue [Line Items]
Operating revenues	36,452	12,089
Transmission Services For Others
Disaggregation of Revenue [Line Items]
Operating revenues	15,249	14,845
Other sources
Disaggregation of Revenue [Line Items]
Operating revenues	4,595	5,916
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 721,000	$ 683,000