DERIVATIVES - Cumulative Basis Adjustment (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Debt securities AFS, carrying amount of hedged asset/liability	$ 72,366	$ 81,082
Debt securities AFS, cumulative fair value hedging adjustment included in the carrying amount, active	103	28
Debt securities AFS, cumulative fair value hedging adjustment included in the carrying amount, de-designated	165	342
Long-term debt, carrying amount of hedged asset/liability	155,659	169,026
Long-term debt, cumulative fair value hedging adjustment included in the carrying amount, active	1,804	5,554
Long-term debt, cumulative fair value hedging adjustment included in the carrying amount, de-designated	4,061	4,989
Cumulative basis adjustment within active hedges	76	(18)
Cumulative basis adjustment within de-designated hedges	(118)	62
Amount of designated hedged items	7,000	3,000
Amortized cost basis of closed portfolios used in hedging relations	$ 30,000	$ 19,000