ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Schedule of Additional Detail on the Cumulative Effect of Change in Accounting Principle (Details) - USD ($) $ in Millions	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 127,528		$ 119,088
ACL Balance	2,210	$ 1,747	1,296
Allowance for loan and lease losses	$ 2,171		$ 1,252
Coverage	1.70%	1.50%	1.10%
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 66,032		$ 57,538
ACL Balance	$ 790	$ 539	$ 718
Coverage	1.20%	0.90%	1.20%
Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 49,092		$ 41,479
ACL Balance	$ 613	$ 376	$ 575
Coverage	1.20%	0.90%	1.40%
Commercial | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 14,502		$ 13,522
ACL Balance	$ 74	$ 67	$ 124
Coverage	0.50%	0.50%	0.90%
Commercial | Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 2,438		$ 2,537
ACL Balance	$ 103	$ 96	$ 19
Coverage	4.20%	3.80%	0.70%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 61,496		$ 61,550
ACL Balance	$ 1,420	$ 1,208	$ 578
Coverage	2.30%	2.00%	0.90%
Retail | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 18,721		$ 19,083
ACL Balance	$ 153	$ 130	$ 35
Coverage	0.80%	0.70%	0.20%
Retail | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 12,992		$ 13,154
ACL Balance	$ 169	$ 156	$ 83
Coverage	1.30%	1.20%	0.60%
Retail | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 12,157		$ 12,120
ACL Balance	$ 278	$ 206	$ 123
Coverage	2.30%	1.70%	1.00%
Retail | Education
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 10,887		$ 10,347
ACL Balance	$ 473	$ 414	$ 116
Coverage	4.30%	4.00%	1.10%
Retail | Other retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 6,739		$ 6,846
ACL Balance	$ 347	$ 302	$ 221
Coverage	5.10%	4.40%	3.20%
Accounting Standards Update 2016-13
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		$ 451
Accounting Standards Update 2016-13 | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		(179)
Accounting Standards Update 2016-13 | Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		(199)
Accounting Standards Update 2016-13 | Commercial | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		(57)
Accounting Standards Update 2016-13 | Commercial | Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		77
Accounting Standards Update 2016-13 | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		630
Accounting Standards Update 2016-13 | Retail | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		95
Accounting Standards Update 2016-13 | Retail | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		73
Accounting Standards Update 2016-13 | Retail | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		83
Accounting Standards Update 2016-13 | Retail | Education
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		298
Accounting Standards Update 2016-13 | Retail | Other retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		$ 81