FAIR VALUE MEASUREMENTS - Schedule of Fair Value Measurements (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Assets
Securities available for sale	[1]	$ 19,958	$ 20,157
Loans held for sale, at fair value		478	497
Derivative assets		369	693
Money market mutual fund		168	165
Other investments		4	4
Total equity securities, at fair value		172	169
Total assets		20,977	21,516
Liabilities
Total derivative liabilities		532	533
Total liabilities		532	533
Interest rate contracts
Assets
Derivative assets		204	538
Liabilities
Total derivative liabilities		377	379
Foreign exchange contracts
Assets
Derivative assets		158	148
Liabilities
Total derivative liabilities		150	149
Other contracts
Assets
Derivative assets		7	7
Liabilities
Total derivative liabilities		5	5
Mortgage-backed securities
Assets
Securities available for sale		19,940	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		289	326
Commercial loans held for sale
Assets
Loans held for sale, at fair value		189	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		168	165
Other investments		0	0
Total equity securities, at fair value		168	165
Total assets		180	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		19,946	20,145
Loans held for sale, at fair value		478	497
Derivative assets		369	693
Money market mutual fund		0	0
Other investments		4	4
Total equity securities, at fair value		4	4
Total assets		20,797	21,339
Liabilities
Total derivative liabilities		532	533
Total liabilities		532	533
Level 2 | Interest rate contracts
Assets
Derivative assets		204	538
Liabilities
Total derivative liabilities		377	379
Level 2 | Foreign exchange contracts
Assets
Derivative assets		158	148
Liabilities
Total derivative liabilities		150	149
Level 2 | Other contracts
Assets
Derivative assets		7	7
Liabilities
Total derivative liabilities		5	5
Level 2 | Mortgage-backed securities
Assets
Securities available for sale		19,940	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		289	326
Level 2 | Commercial loans held for sale
Assets
Loans held for sale, at fair value		189	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		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.