SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Reclassifications Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 19,472
Ending balance	$ 19,696		19,696
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(13)	$ (8)	(8)	$ (9)
Net current period other comprehensive gain (loss)	1	0	(4)	1
Ending balance	(12)	(8)	(12)	(8)
AOCI Attributable to Parent | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(30)	(25)	(25)	(25)
Net current period other comprehensive gain (loss)	1	0	(4)	0
Ending balance	(29)	(25)	(29)	(25)
AOCI Attributable to Parent | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	17	17	17	16
Net current period other comprehensive gain (loss)	0	0	0	1
Ending balance	17	17	17	17
Amortization of prior service benefit (cost)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	2	1	5	4
Amortization of prior service benefit (cost) | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(1)	(1)	(3)	(3)
Amount attributable to tax	0	0	1	2
Amortization of prior service benefit (cost) | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	3	2	8	7
Amount attributable to tax	1	2	3	5
Amortization of net actuarial gain (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	0	5	0	12
Amortization of net actuarial gain (loss) | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	1	4	3	10
Amount attributable to tax	0	2	1	7
Amortization of net actuarial gain (loss) | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(1)	1	(3)	2
Amount attributable to tax	0	0	1	1
Regulatory account transfer
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(1)	(6)	(4)	(15)
Regulatory account transfer | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	1	(3)	1	(7)
Amount attributable to tax	0	2	0	5
Regulatory account transfer | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(2)	(3)	(5)	(8)
Amount attributable to tax	$ 1	$ 2	2	$ 6
Reclassification of stranded income tax to retained earnings
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income			(5)
Reclassification of stranded income tax to retained earnings | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income			(5)
Reclassification of stranded income tax to retained earnings | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income			$ 0