INVESTMENT SECURITIES - Schedule of Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Gross unrealized gains	$ 7,677	$ 5,736
Gross unrealized losses	(202)	(31)
Equity	5,318	10,025
Amortized cost	39,406	42,816
Estimated fair value	46,881	48,521
U.S. corporate
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost	23,649	23,037
Gross unrealized gains	6,090	4,636
Gross unrealized losses	(83)	(11)
Estimated fair value	29,656	27,661
Non-U.S. corporate
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost	2,290	2,161
Gross unrealized gains	364	260
Gross unrealized losses	(2)	(1)
Estimated fair value	2,652	2,420
State and municipal
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost	3,322	3,086
Gross unrealized gains	878	598
Gross unrealized losses	(18)	(15)
Estimated fair value	4,182	3,669
Mortgage and asset-backed
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost	3,561	3,117
Gross unrealized gains	147	116
Gross unrealized losses	(100)	(4)
Estimated fair value	3,609	3,229
Government and agencies
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost	1,266	1,391
Gross unrealized gains	198	126
Gross unrealized losses	0	0
Estimated fair value	$ 1,464	$ 1,516