SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Reclassifications Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 19,472
Ending balance	19,983
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(8)	$ (9)
Net current period other comprehensive gain (loss)	(5)	0
Ending balance	(13)	(9)
AOCI Attributable to Parent | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(25)	(25)
Net current period other comprehensive gain (loss)	(5)	0
Ending balance	(30)	(25)
AOCI Attributable to Parent | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	17	16
Net current period other comprehensive gain (loss)	0	0
Ending balance	17	16
Amortization of prior service cost (net of taxes of $0 and $1, respectively) (1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	2	1
Amortization of prior service cost (net of taxes of $0 and $1, respectively) (1) | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(1)	(1)
Amount attributable to tax	0	1
Amortization of prior service cost (net of taxes of $0 and $1, respectively) (1) | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	3	2
Amount attributable to tax	1	2
Amortization of net actuarial loss (net of taxes of $0 and $0, respectively) (1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	0	4
Amortization of net actuarial loss (net of taxes of $0 and $0, respectively) (1) | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	1	3
Amount attributable to tax	0	3
Amortization of net actuarial loss (net of taxes of $0 and $0, respectively) (1) | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(1)	1
Amount attributable to tax	0	0
Regulatory account transfer (net of taxes of $0 and $1, respectively) (1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(2)	(5)
Regulatory account transfer (net of taxes of $0 and $1, respectively) (1) | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	0	(2)
Amount attributable to tax	0	2
Regulatory account transfer (net of taxes of $0 and $1, respectively) (1) | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(2)	(3)
Amount attributable to tax	1	$ 2
Reclassification of stranded income tax to retained earnings (net of taxes of $0 and $0, respectively)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(5)
Reclassification of stranded income tax to retained earnings (net of taxes of $0 and $0, respectively) | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(5)
Amount attributable to tax	0
Reclassification of stranded income tax to retained earnings (net of taxes of $0 and $0, respectively) | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	0
Amount attributable to tax	$ 0