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	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 125,713		$ 119,088
ACL Balance	$ 2,527	$ 1,747	$ 1,296
Coverage	2.00%	1.50%	1.10%
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 64,930		$ 57,538
ACL Balance	$ 1,304	$ 539	$ 718
Coverage	2.00%	0.90%	1.20%
Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 48,017		$ 41,479
ACL Balance	$ 847	$ 376	$ 575
Coverage	1.80%	0.90%	1.40%
Commercial | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 14,485		$ 13,522
ACL Balance	$ 330	$ 67	$ 124
Coverage	2.30%	0.50%	0.90%
Commercial | Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 2,428		$ 2,537
ACL Balance	$ 127	$ 96	$ 19
Coverage	5.20%	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	0.19%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 60,783		$ 61,550
ACL Balance	$ 1,223	$ 1,208	$ 578
Coverage	2.00%	2.00%	0.90%
Retail | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 19,245		$ 19,083
ACL Balance	$ 104	$ 130	$ 35
Coverage	0.50%	0.70%	0.20%
Retail | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 12,541		$ 13,154
ACL Balance	$ 143	$ 156	$ 83
Coverage	1.10%	1.20%	0.60%
Retail | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 12,028		$ 12,120
ACL Balance	$ 277	$ 206	$ 123
Coverage	2.30%	1.70%	1.00%
Retail | Education
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 10,591		$ 10,347
ACL Balance	$ 312	$ 414	$ 116
Coverage	2.90%	4.00%	1.10%
Retail | Other retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 6,378		$ 6,846
ACL Balance	$ 387	$ 302	$ 221
Coverage	6.10%	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