GUARANTEES AND COMMITMENTS - Performance Risk (Details) - USD ($) $ in Billions	Mar. 31, 2018	Dec. 31, 2017
Guarantor Obligations
Maximum potential amount of future payments	$ 457.0	$ 427.8
Investment grade
Guarantor Obligations
Maximum potential amount of future payments	102.1	99.7
Non-investment grade
Guarantor Obligations
Maximum potential amount of future payments	25.3	25.6
Not rated
Guarantor Obligations
Maximum potential amount of future payments	329.6	302.5
Financial standby letters of credit
Guarantor Obligations
Maximum potential amount of future payments	93.4	93.8
Financial standby letters of credit | Investment grade
Guarantor Obligations
Maximum potential amount of future payments	68.1	68.1
Financial standby letters of credit | Non-investment grade
Guarantor Obligations
Maximum potential amount of future payments	10.3	10.9
Financial standby letters of credit | Not rated
Guarantor Obligations
Maximum potential amount of future payments	15.0	14.8
Performance guarantees
Guarantor Obligations
Maximum potential amount of future payments	11.6	11.3
Performance guarantees | Investment grade
Guarantor Obligations
Maximum potential amount of future payments	8.2	7.9
Performance guarantees | Non-investment grade
Guarantor Obligations
Maximum potential amount of future payments	2.3	2.4
Performance guarantees | Not rated
Guarantor Obligations
Maximum potential amount of future payments	1.1	1.0
Derivative instruments deemed to be guarantees
Guarantor Obligations
Maximum potential amount of future payments	102.6	95.9
Derivative instruments deemed to be guarantees | Investment grade
Guarantor Obligations
Maximum potential amount of future payments	0.0	0.0
Derivative instruments deemed to be guarantees | Non-investment grade
Guarantor Obligations
Maximum potential amount of future payments	0.0	0.0
Derivative instruments deemed to be guarantees | Not rated
Guarantor Obligations
Maximum potential amount of future payments	102.6	95.9
Loans sold with recourse
Guarantor Obligations
Maximum potential amount of future payments	0.2	0.2
Loans sold with recourse | Investment grade
Guarantor Obligations
Maximum potential amount of future payments	0.0	0.0
Loans sold with recourse | Non-investment grade
Guarantor Obligations
Maximum potential amount of future payments	0.0	0.0
Loans sold with recourse | Not rated
Guarantor Obligations
Maximum potential amount of future payments	0.2	0.2
Securities lending indemnifications
Guarantor Obligations
Maximum potential amount of future payments	126.9	103.7
Securities lending indemnifications | Investment grade
Guarantor Obligations
Maximum potential amount of future payments	0.0	0.0
Securities lending indemnifications | Non-investment grade
Guarantor Obligations
Maximum potential amount of future payments	0.0	0.0
Securities lending indemnifications | Not rated
Guarantor Obligations
Maximum potential amount of future payments	126.9	103.7
Credit card merchant processing
Guarantor Obligations
Maximum potential amount of future payments	82.5	85.5
Credit card merchant processing | Investment grade
Guarantor Obligations
Maximum potential amount of future payments	0.0	0.0
Credit card merchant processing | Non-investment grade
Guarantor Obligations
Maximum potential amount of future payments	0.0	0.0
Credit card merchant processing | Not rated
Guarantor Obligations
Maximum potential amount of future payments	82.5	85.5
Credit card arrangements with partners
Guarantor Obligations
Maximum potential amount of future payments	1.3	1.4
Credit card arrangements with partners | Investment grade
Guarantor Obligations
Maximum potential amount of future payments	0.0	0.0
Credit card arrangements with partners | Non-investment grade
Guarantor Obligations
Maximum potential amount of future payments	0.0	0.0
Credit card arrangements with partners | Not rated
Guarantor Obligations
Maximum potential amount of future payments	1.3	1.4
Custody indemnifications and other
Guarantor Obligations
Maximum potential amount of future payments	38.5	36.0
Custody indemnifications and other | Investment grade
Guarantor Obligations
Maximum potential amount of future payments	25.8	23.7
Custody indemnifications and other | Non-investment grade
Guarantor Obligations
Maximum potential amount of future payments	12.7	12.3
Custody indemnifications and other | Not rated
Guarantor Obligations
Maximum potential amount of future payments	$ 0.0	$ 0.0