SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Reclassifications Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 5,388	$ 12,903
Net current period other comprehensive gain (loss)	0	0
Ending balance	5,759	13,129
Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net current period other comprehensive gain (loss)	0	0
Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net current period other comprehensive gain (loss)	0	0
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(5)	(4)
Ending balance	(5)	(4)
Accumulated Other Comprehensive Income (Loss) | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(22)	(21)
Ending balance	(22)	(21)
Accumulated Other Comprehensive Income (Loss) | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	17	17
Ending balance	17	17
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	1	2
Amortization of prior service cost | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(1)	(1)
Amount attributable to tax	0	0
Amortization of prior service cost | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	2	3
Amount attributable to tax	1	1
Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(2)	0
Amortization of net actuarial loss | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	1	1
Amount attributable to tax	0	0
Amortization of net actuarial loss | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(3)	(1)
Amount attributable to tax	2	0
Regulatory account transfer
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	1	(2)
Regulatory account transfer | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	0	0
Amount attributable to tax	0	0
Regulatory account transfer | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	1	(2)
Amount attributable to tax	$ 1	$ 1