NONINTEREST INCOME - Components of Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 310	$ 227	$ 595	$ 521
Total noninterest income	485	590	1,027	1,087
Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue from other sources	175	363	432	566
Total noninterest income	485	590	1,027	1,087
Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	190	146	369	336
Consumer Banking | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue from other sources	93	282	265	449
Total noninterest income	283	428	634	785
Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	120	81	226	185
Commercial Banking | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue from other sources	58	63	122	84
Total noninterest income	178	144	348	269
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other | Operating Segments
Disaggregation of Revenue [Line Items]
Total revenue from other sources	24	18	45	33
Total noninterest income	24	18	45	33
Service charges and fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	100	84	199	202
Service charges and fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	74	59	148	151
Service charges and fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	26	25	51	51
Service charges and fees | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Card fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	64	49	118	104
Card fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	56	42	103	87
Card fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	8	7	15	17
Card fees | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Capital markets fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	84	48	156	113
Capital markets fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Capital markets fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	84	48	156	113
Capital markets fees | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Trust and investment services fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	60	45	118	98
Trust and investment services fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	60	45	118	98
Trust and investment services fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Trust and investment services fees | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2	1	4	4
Other banking fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other banking fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2	1	4	4
Other banking fees | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 0	$ 0	$ 0	$ 0