Revenue (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Operating revenues	$ 974,123	$ 1,666,837
Regulatory cost recovery revenue	20,000	27,000
Electric Service | Retail residential electric service
Disaggregation of Revenue [Line Items]
Operating revenues	500,247	816,922
Electric Service | Retail non-residential electric service
Disaggregation of Revenue [Line Items]
Operating revenues	435,500	778,689
Electric Service | Wholesale energy sales
Disaggregation of Revenue [Line Items]
Operating revenues	15,392	27,481
Transmission Services For Others
Disaggregation of Revenue [Line Items]
Operating revenues	15,489	30,334
Other sources
Disaggregation of Revenue [Line Items]
Operating revenues	7,495	13,411
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 954,000	$ 1,640,000