COMMISSIONS AND FEES; ADMINISTRATION AND OTHER FIDUCIARY FEES - Administration and Other Fiduciary Fees (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commissions and fees
Administration and other fiduciary fees	$ 911	$ 929	$ 2,750	$ 2,672
Custody fees
Commissions and fees
Administration and other fiduciary fees	430	455	1,321	1,299
Custody fees | ICG
Commissions and fees
Administration and other fiduciary fees	371	397	1,138	1,135
Custody fees | GCB
Commissions and fees
Administration and other fiduciary fees	41	44	133	123
Custody fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	18	14	50	41
Fiduciary fees
Commissions and fees
Administration and other fiduciary fees	330	324	978	927
Fiduciary fees | ICG
Commissions and fees
Administration and other fiduciary fees	160	149	492	437
Fiduciary fees | GCB
Commissions and fees
Administration and other fiduciary fees	158	157	455	431
Fiduciary fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	12	18	31	59
Guarantee fees
Commissions and fees
Administration and other fiduciary fees	151	150	451	446
Guarantee fees | ICG
Commissions and fees
Administration and other fiduciary fees	136	134	403	400
Guarantee fees | GCB
Commissions and fees
Administration and other fiduciary fees	14	13	43	39
Guarantee fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	1	3	5	7
Administration and other fiduciary fees
Commissions and fees
Administration and other fiduciary fees	911	929	2,750	2,672
Revenue not accounted for under ASC 606, Revenue from Contracts with Customers	151	150	451	446
Administration and other fiduciary fees | ICG
Commissions and fees
Administration and other fiduciary fees	667	680	2,033	1,972
Administration and other fiduciary fees | GCB
Commissions and fees
Administration and other fiduciary fees	213	214	631	593
Administration and other fiduciary fees | Corporate/Other
Commissions and fees
Administration and other fiduciary fees	$ 31	$ 35	$ 86	$ 107