FAIR VALUE MEASUREMENTS - Schedule of Fair Value Measurements (Details) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Assets
Securities available for sale	[1]	$ 20,157	$ 20,157
Loans held for sale, at fair value		521	497
Derivative assets		357	693
Money market mutual fund investments		170	165
Other investments		0	4
Total equity securities, at fair value		170	169
Total assets		21,205	21,516
Liabilities
Total derivative liabilities		568	533
Total liabilities		568	533
Interest rate contracts
Assets
Derivative assets		206	538
Liabilities
Total derivative liabilities		436	379
Foreign exchange contracts
Assets
Derivative assets		143	148
Liabilities
Total derivative liabilities		126	149
Other contracts
Assets
Derivative assets		8	7
Liabilities
Total derivative liabilities		6	5
Mortgage-backed securities
Assets
Securities available for sale		20,139	20,139
State and political subdivisions
Assets
Securities available for sale		6	6
U.S. Treasury and other
Assets
Securities available for sale		12	12
Residential loans held for sale
Assets
Loans held for sale, at fair value		365	326
Commercial loans held for sale
Assets
Loans held for sale, at fair value		156	171
Level 1
Assets
Securities available for sale		12	12
Loans held for sale, at fair value		0	0
Derivative assets		0	0
Money market mutual fund investments		170	165
Other investments		0	0
Total equity securities, at fair value		170	165
Total assets		182	177
Liabilities
Total derivative liabilities		0	0
Total liabilities		0	0
Level 1 | Interest rate contracts
Assets
Derivative assets		0	0
Liabilities
Total derivative liabilities		0	0
Level 1 | Foreign exchange contracts
Assets
Derivative assets		0	0
Liabilities
Total derivative liabilities		0	0
Level 1 | Other contracts
Assets
Derivative assets		0	0
Liabilities
Total derivative liabilities		0	0
Level 1 | Mortgage-backed securities
Assets
Securities available for sale		0	0
Level 1 | State and political subdivisions
Assets
Securities available for sale		0	0
Level 1 | U.S. Treasury and other
Assets
Securities available for sale		12	12
Level 1 | Residential loans held for sale
Assets
Loans held for sale, at fair value		0	0
Level 1 | Commercial loans held for sale
Assets
Loans held for sale, at fair value		0	0
Level 2
Assets
Securities available for sale		20,145	20,145
Loans held for sale, at fair value		521	497
Derivative assets		357	693
Money market mutual fund investments		0	0
Other investments		0	4
Total equity securities, at fair value		0	4
Total assets		21,023	21,339
Liabilities
Total derivative liabilities		568	533
Total liabilities		568	533
Level 2 | Interest rate contracts
Assets
Derivative assets		206	538
Liabilities
Total derivative liabilities		436	379
Level 2 | Foreign exchange contracts
Assets
Derivative assets		143	148
Liabilities
Total derivative liabilities		126	149
Level 2 | Other contracts
Assets
Derivative assets		8	7
Liabilities
Total derivative liabilities		6	5
Level 2 | Mortgage-backed securities
Assets
Securities available for sale		20,139	20,139
Level 2 | State and political subdivisions
Assets
Securities available for sale		6	6
Level 2 | U.S. Treasury and other
Assets
Securities available for sale		0	0
Level 2 | Residential loans held for sale
Assets
Loans held for sale, at fair value		365	326
Level 2 | Commercial loans held for sale
Assets
Loans held for sale, at fair value		156	171
Level 3
Assets
Securities available for sale		0	0
Loans held for sale, at fair value		0	0
Derivative assets		0	0
Money market mutual fund investments		0	0
Other investments		0	0
Total equity securities, at fair value		0	0
Total assets		0	0
Liabilities
Total derivative liabilities		0	0
Total liabilities		0	0
Level 3 | Interest rate contracts
Assets
Derivative assets		0	0
Liabilities
Total derivative liabilities		0	0
Level 3 | Foreign exchange contracts
Assets
Derivative assets		0	0
Liabilities
Total derivative liabilities		0	0
Level 3 | Other contracts
Assets
Derivative assets		0	0
Liabilities
Total derivative liabilities		0	0
Level 3 | Mortgage-backed securities
Assets
Securities available for sale		0	0
Level 3 | State and political subdivisions
Assets
Securities available for sale		0	0
Level 3 | U.S. Treasury and other
Assets
Securities available for sale		0	0
Level 3 | Residential loans held for sale
Assets
Loans held for sale, at fair value		0	0
Level 3 | Commercial loans held for sale
Assets
Loans held for sale, at fair value		$ 0	$ 0

[1] Includes only collateral pledged by the Company where counterparties have the right to sell or pledge the collateral.