SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Reclassifications Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 19,472
Ending balance	$ 19,061		19,061
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(13)	$ (9)	(8)	$ (9)
Net current period other comprehensive gain (loss)	0	1	(5)	1
Ending balance	(13)	(8)	(13)	(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)	0	0	(5)	0
Ending balance	(30)	(25)	(30)	(25)
AOCI Attributable to Parent | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	17	16	17	16
Net current period other comprehensive gain (loss)	0	1	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	1	2	3	3
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)	(2)	(2)
Amount attributable to tax	1	1	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	2	3	5	5
Amount attributable to tax	1	1	2	3
Amortization of net actuarial gain (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	0	3	0	7
Amortization of net actuarial gain (loss) | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	1	3	2	6
Amount attributable to tax	1	2	1	5
Amortization of net actuarial gain (loss) | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(1)	0	(2)	1
Amount attributable to tax	1	1	1	1
Regulatory account transfer
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(1)	(4)	(3)	(9)
Regulatory account transfer | Pension Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	0	(2)	0	(4)
Amount attributable to tax	0	1	0	3
Regulatory account transfer | Other Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	(1)	(2)	(3)	(5)
Amount attributable to tax	0	$ 2	1	$ 4
Reclassification of stranded income tax to retained earnings
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income	0		(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	0		(5)
Amount attributable to tax	0		0
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		0
Amount attributable to tax	$ 0		$ 0