COMMISSIONS AND FEES; ADMINISTRATION AND OTHER FIDUCIARY FEES - Administration and Other Fiduciary Fees (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Commissions and fees
Administration and other fiduciary fees	$ 854	$ 839
Custody fees
Commissions and fees
Administration and other fiduciary fees	389	383
Custody fees | ICG
Commissions and fees
Administration and other fiduciary fees	366	364
Custody fees | GCB
Commissions and fees
Administration and other fiduciary fees	8	3
Custody fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	15	16
Fiduciary fees
Commissions and fees
Administration and other fiduciary fees	328	310
Fiduciary fees | ICG
Commissions and fees
Administration and other fiduciary fees	172	152
Fiduciary fees | GCB
Commissions and fees
Administration and other fiduciary fees	156	146
Fiduciary fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	0	12
Guarantee fees
Commissions and fees
Administration and other fiduciary fees	137	146
Guarantee fees | ICG
Commissions and fees
Administration and other fiduciary fees	134	142
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	2
Administration and other fiduciary fees
Commissions and fees
Administration and other fiduciary fees	854	839
Revenue not accounted for under ASC 606, revenue from contracts with customers	136	146
Administration and other fiduciary fees | ICG
Commissions and fees
Administration and other fiduciary fees	672	658
Administration and other fiduciary fees | GCB
Commissions and fees
Administration and other fiduciary fees	166	151
Administration and other fiduciary fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	$ 16	$ 30