SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Reclassifications Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ (5)	$ (4)
Net current period other comprehensive loss	(17)	(1)
Ending balance	(22)	(5)
Accumulated Other Comprehensive Loss | Pension Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(22)	(21)
Net current period other comprehensive loss	(17)	(1)
Ending balance	(39)	(22)
Accumulated Other Comprehensive Loss | PBOP Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	17	17
Net current period other comprehensive loss	0	0
Ending balance	17	17
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income:	6	6
Amortization of prior service cost | Pension Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income:	(4)	(4)
Amounts reclassified from other comprehensive income, tax	2	2
Amortization of prior service cost | PBOP Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from other comprehensive income:	10	10
Amounts reclassified from other comprehensive income, tax	4	4
Amortization of net actuarial gain (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications:	(247)	288
Amounts reclassified from other comprehensive income:	(13)	0
Amortization of net actuarial gain (loss) | Pension Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications:	(417)	61
Amounts reclassified from other comprehensive income:	2	2
Other comprehensive income before reclassifications, tax	162	24
Amounts reclassified from other comprehensive income, tax	1	1
Amortization of net actuarial gain (loss) | PBOP Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications:	170	227
Amounts reclassified from other comprehensive income:	(15)	(2)
Other comprehensive income before reclassifications, tax	66	88
Amounts reclassified from other comprehensive income, tax	6	1
Regulatory account transfer
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications:	230	(289)
Amounts reclassified from other comprehensive income:	7	(6)
Regulatory account transfer | Pension Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications:	400	(62)
Amounts reclassified from other comprehensive income:	2	2
Other comprehensive income before reclassifications, tax	155	24
Amounts reclassified from other comprehensive income, tax	1	1
Regulatory account transfer | PBOP Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications:	(170)	(227)
Amounts reclassified from other comprehensive income:	5	(8)
Other comprehensive income before reclassifications, tax	66	88
Amounts reclassified from other comprehensive income, tax	$ 2	$ 3