LOANS - Corporate Troubled Debt Restructurings (Details) - Corporate - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financing receivable impaired
Carrying Value	$ 189	$ 124	$ 496	$ 475
TDRs involving changes in the amount and/or timing of principal payments	99	114	131	187
TDRs involving changes in the amount and/or timing of interest payments	0	2	0	176
TDRs involving changes in the amount and/or timing of both principal and interest payments	90	8	$ 365	112
Period within which default occurred post-modification			1 year
Number of days past due, default status			60 days
Carrying Value	949	775	$ 949	775
TDR in payment default	0	0	15	7
Commercial and industrial
Financing receivable impaired
Carrying Value	175	112	463	316
TDRs involving changes in the amount and/or timing of principal payments	99	103	131	176
TDRs involving changes in the amount and/or timing of interest payments	0	2	0	34
TDRs involving changes in the amount and/or timing of both principal and interest payments	76	7	332	106
Carrying Value	686	394	686	394
TDR in payment default	0	0	12	7
Financial institutions
Financing receivable impaired
Carrying Value		10	15	10
TDRs involving changes in the amount and/or timing of principal payments		10	0	10
TDRs involving changes in the amount and/or timing of interest payments		0	0	0
TDRs involving changes in the amount and/or timing of both principal and interest payments		0	15	0
Carrying Value	24	10	24	10
TDR in payment default	0	0	3	0
Mortgage and real estate
Financing receivable impaired
Carrying Value	14	2	18	7
TDRs involving changes in the amount and/or timing of principal payments	0	1	0	1
TDRs involving changes in the amount and/or timing of interest payments	0	0	0	0
TDRs involving changes in the amount and/or timing of both principal and interest payments	14	1	18	6
Carrying Value	84	80	84	80
TDR in payment default	0	0	0	0
Other
Financing receivable impaired
Carrying Value				142
TDRs involving changes in the amount and/or timing of principal payments				0
TDRs involving changes in the amount and/or timing of interest payments				142
TDRs involving changes in the amount and/or timing of both principal and interest payments				0
Carrying Value	155	291	155	291
TDR in payment default	$ 0	$ 0	$ 0	$ 0
Commercial banking
Financing receivable impaired
Number of days past due, default status			90 days