Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Operating revenues	$ 869,501	$ 974,123	$ 1,610,031	$ 1,666,837
Regulatory cost recovery revenue	12,000	20,000	32,000	27,000
Electric Service | Retail residential electric service
Disaggregation of Revenue [Line Items]
Operating revenues	432,568	500,247	784,134	816,922
Electric Service | Retail non-residential electric service
Disaggregation of Revenue [Line Items]
Operating revenues	395,929	435,500	728,597	778,689
Electric Service | Wholesale energy sales
Disaggregation of Revenue [Line Items]
Operating revenues	21,991	15,392	58,443	27,481
Transmission Services For Others
Disaggregation of Revenue [Line Items]
Operating revenues	15,157	15,489	30,406	30,334
Other sources
Disaggregation of Revenue [Line Items]
Operating revenues	3,856	7,495	8,451	13,411
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 858,000	$ 954,000	$ 1,578,000	$ 1,640,000