COMMISSIONS AND FEES; ADMINISTRATION AND OTHER FIDUCIARY FEES - Administration and Other Fiduciary Fees (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Commissions and fees
Administration and other fiduciary fees	$ 961	$ 854
Custody fees
Commissions and fees
Administration and other fiduciary fees	457	389
Custody fees | ICG
Commissions and fees
Administration and other fiduciary fees	451	366
Custody fees | GCB
Commissions and fees
Administration and other fiduciary fees	6	8
Custody fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	0	15
Fiduciary fees
Commissions and fees
Administration and other fiduciary fees	359	328
Fiduciary fees | ICG
Commissions and fees
Administration and other fiduciary fees	192	172
Fiduciary fees | GCB
Commissions and fees
Administration and other fiduciary fees	167	156
Fiduciary fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	0	0
Guarantee fees
Commissions and fees
Administration and other fiduciary fees	145	137
Guarantee fees | ICG
Commissions and fees
Administration and other fiduciary fees	142	134
Guarantee fees | GCB
Commissions and fees
Administration and other fiduciary fees	2	2
Guarantee fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	1	1
Administration and other fiduciary fees
Commissions and fees
Administration and other fiduciary fees	961	854
Revenue not accounted for under ASC 606, revenue from contracts with customers	145	136
Administration and other fiduciary fees | ICG
Commissions and fees
Administration and other fiduciary fees	785	672
Administration and other fiduciary fees | GCB
Commissions and fees
Administration and other fiduciary fees	175	166
Administration and other fiduciary fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	$ 1	$ 16