SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 128	$ 118
Interest cost	172	179
Expected return on plan assets	(255)	(193)
Amortization of prior service cost	(1)	(2)
Amortization of net actuarial loss	1	6
Net periodic benefit cost	45	108
Regulatory account transfer	39	(23)
Total	84	85
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	16	15
Interest cost	17	19
Expected return on plan assets	(33)	(24)
Amortization of prior service cost	4	4
Amortization of net actuarial loss	(1)	1
Net periodic benefit cost	3	15
Regulatory account transfer	0	0
Total	$ 3	$ 15