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	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 124,071		$ 119,088
ACL Balance	$ 2,736	$ 1,747	$ 1,296
Coverage	2.20%	1.50%	1.10%
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 62,362		$ 57,538
ACL Balance	$ 1,441	$ 539	$ 718
Coverage	2.30%	0.90%	1.20%
Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 45,185		$ 41,479
ACL Balance	$ 826	$ 376	$ 575
Coverage	1.80%	0.90%	1.40%
Commercial | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 14,889		$ 13,522
ACL Balance	$ 548	$ 67	$ 124
Coverage	3.70%	0.50%	0.90%
Commercial | Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 2,288		$ 2,537
ACL Balance	$ 67	$ 96	$ 19
Coverage	2.90%	3.80%	0.70%
Commercial | SBA Guaranteed PPP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 4,700
Coverage reduction, basis points	0.21%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 61,709		$ 61,550
ACL Balance	$ 1,295	$ 1,208	$ 578
Coverage	2.10%	2.00%	0.90%
Retail | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 19,633		$ 19,083
ACL Balance	$ 133	$ 130	$ 35
Coverage	0.70%	0.70%	0.20%
Retail | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 12,322		$ 13,154
ACL Balance	$ 156	$ 156	$ 83
Coverage	1.30%	1.20%	0.60%
Retail | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 12,035		$ 12,120
ACL Balance	$ 221	$ 206	$ 123
Coverage	1.80%	1.70%	1.00%
Retail | Education
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 11,631		$ 10,347
ACL Balance	$ 386	$ 414	$ 116
Coverage	3.30%	4.00%	1.10%
Retail | Other retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 6,088		$ 6,846
ACL Balance	$ 399	$ 302	$ 221
Coverage	6.60%	4.40%	3.20%
Accounting Standards Update 2016-13
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		$ 451
Accounting Standards Update 2016-13 | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		(179)
Accounting Standards Update 2016-13 | Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		(199)
Accounting Standards Update 2016-13 | Commercial | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		(57)
Accounting Standards Update 2016-13 | Commercial | Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		77
Accounting Standards Update 2016-13 | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		630
Accounting Standards Update 2016-13 | Retail | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		95
Accounting Standards Update 2016-13 | Retail | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		73
Accounting Standards Update 2016-13 | Retail | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		83
Accounting Standards Update 2016-13 | Retail | Education
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		298
Accounting Standards Update 2016-13 | Retail | Other retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
ACL Balance		$ 81