OTHER COMPREHENSIVE INCOME - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 5,037	$ 4,897	$ 4,959	$ 4,883
Other comprehensive (loss) income before reclassifications			(24)
Amounts reclassified from AOCI			11
Other Comprehensive Income (Loss)	(6)	9	(13)	39
Balance at end of period	5,098	4,951	5,098	4,951
Unrealized Net Gains (Losses) on Debt Securities Available for Sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			65
Other comprehensive (loss) income before reclassifications			(29)
Amounts reclassified from AOCI			0
Other Comprehensive Income (Loss)			(29)
Balance at end of period	36		36
Unrealized Net Gains (Losses) on Derivative Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(40)
Other comprehensive (loss) income before reclassifications			3
Amounts reclassified from AOCI			11
Other Comprehensive Income (Loss)			14
Balance at end of period	(26)		(26)
Unrecognized Pension and Postretirement Obligations [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(64)
Other comprehensive (loss) income before reclassifications			2
Amounts reclassified from AOCI			0
Other Comprehensive Income (Loss)			2
Balance at end of period	(62)		(62)
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(46)	(35)	(39)	(65)
Balance at end of period	$ (52)	$ (26)	$ (52)	$ (26)