INVESTMENT SECURITIES - Schedule of Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Gross unrealized gains	$ 7,237	$ 5,736
Gross unrealized losses	(300)	(31)
Equity	6,465	10,025
Amortized cost	40,179	42,816
Estimated fair value	47,117	48,521
U.S. corporate
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost	23,540	23,037
Gross unrealized gains	5,771	4,636
Gross unrealized losses	(136)	(11)
Estimated fair value	29,175	27,661
Non-U.S. corporate
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost	2,185	2,161
Gross unrealized gains	337	260
Gross unrealized losses	(5)	(1)
Estimated fair value	2,517	2,420
State and municipal
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost	3,235	3,086
Gross unrealized gains	826	598
Gross unrealized losses	(22)	(15)
Estimated fair value	4,039	3,669
Mortgage and asset-backed
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost	3,497	3,117
Gross unrealized gains	97	116
Gross unrealized losses	(136)	(4)
Estimated fair value	3,458	3,229
Government and agencies
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost	1,257	1,391
Gross unrealized gains	206	126
Gross unrealized losses	0	0
Estimated fair value	$ 1,463	$ 1,516