SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Revenues Disaggregated by Type of Customer) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue from contracts with customers
Total operating revenues	$ 4,056	$ 4,268
Electric
Revenue from contracts with customers
Total revenue from contracts with customers	2,677
Regulatory balancing accounts	274
Total operating revenues	2,951
Electric | Residential
Revenue from contracts with customers
Total revenue from contracts with customers	1,336
Electric | Commercial
Revenue from contracts with customers
Total revenue from contracts with customers	1,073
Electric | Industrial
Revenue from contracts with customers
Total revenue from contracts with customers	324
Electric | Agricultural
Revenue from contracts with customers
Total revenue from contracts with customers	125
Electric | Public street and highway lighting
Revenue from contracts with customers
Total revenue from contracts with customers	20
Electric | Other
Revenue from contracts with customers
Total revenue from contracts with customers	(201)
Natural gas
Revenue from contracts with customers
Total revenue from contracts with customers	1,399
Regulatory balancing accounts	(294)
Total operating revenues	1,105
Natural gas | Residential
Revenue from contracts with customers
Total revenue from contracts with customers	958
Natural gas | Commercial
Revenue from contracts with customers
Total revenue from contracts with customers	196
Natural gas | Transportation service only
Revenue from contracts with customers
Total revenue from contracts with customers	297
Natural gas | Other
Revenue from contracts with customers
Total revenue from contracts with customers	$ (52)