INVESTMENT SECURITIES - Schedule of Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Gross unrealized gains	$ 3,608		$ 2,792
Gross unrealized losses (a)	(134)		(561)
Equity	3,639		146
Amortized cost	34,802		31,605
Estimated fair value	38,275		33,835
Net unrealized gains (losses) recorded to earnings	19	$ (29)
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	21,390		21,306
Gross unrealized gains	2,903		2,257
Gross unrealized losses (a)	(71)		(357)
Estimated fair value	24,222		23,206
Non-U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,848		1,906
Gross unrealized gains	96		53
Gross unrealized losses (a)	(12)		(76)
Estimated fair value	1,932		1,883
State and municipal
Debt Securities, Available-for-sale [Line Items]
Amortized cost	3,136		3,320
Gross unrealized gains	455		367
Gross unrealized losses (a)	(37)		(54)
Estimated fair value	3,555		3,633
Mortgage and asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost	3,184		3,325
Gross unrealized gains	75		51
Gross unrealized losses (a)	(15)		(54)
Estimated fair value	3,244		3,322
Government and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,604		1,603
Gross unrealized gains	79		63
Gross unrealized losses (a)	(1)		(20)
Estimated fair value	$ 1,682		$ 1,645