FAIR VALUE MEASUREMENTS - Schedule of Fair Value Measurements (Details) - USD ($) $ in Millions		Dec. 31, 2017		Dec. 31, 2016
Assets
Securities available for sale	[1]	$ 20,157		$ 19,501
Loans held for sale		497		583
Derivative assets		693	[2]	759
Other investment securities, at fair value:
Money market mutual fund		165		91
Other investments		4		5
Total other investment securities, at fair value		169		96
Total assets		21,516		20,939
Liabilities
Total derivative liabilities		533	[2]	778
Total liabilities		533		778
Interest rate swaps
Assets
Derivative assets		538		609
Liabilities
Total derivative liabilities		379		645
Foreign exchange contracts
Assets
Derivative assets		148		134
Liabilities
Total derivative liabilities		149		126
Other contracts
Assets
Derivative assets		7		16
Liabilities
Total derivative liabilities		5		7
Mortgage-backed securities
Assets
Securities available for sale		20,139		19,446
State and political subdivisions
Assets
Securities available for sale		6		8
Equity securities
Assets
Securities available for sale				17
U.S. Treasury and other
Assets
Securities available for sale		12		30
Residential loans held for sale
Assets
Loans held for sale		326		504
Commercial loans held for sale
Assets
Loans held for sale		171		79
Level 1
Assets
Securities available for sale		12		30
Loans held for sale		0		0
Derivative assets		0		0
Other investment securities, at fair value:
Money market mutual fund		165		91
Other investments		0		0
Total other investment securities, at fair value		165		91
Total assets		177		121
Liabilities
Total derivative liabilities		0		0
Total liabilities		0		0
Level 1 | Interest rate swaps
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 | Equity securities
Assets
Securities available for sale				0
Level 1 | U.S. Treasury and other
Assets
Securities available for sale		12		30
Level 1 | Residential loans held for sale
Assets
Loans held for sale		0		0
Level 1 | Commercial loans held for sale
Assets
Loans held for sale		0		0
Level 2
Assets
Securities available for sale		20,145		19,471
Loans held for sale		497		583
Derivative assets		693		759
Other investment securities, at fair value:
Money market mutual fund		0		0
Other investments		4		5
Total other investment securities, at fair value		4		5
Total assets		21,339		20,818
Liabilities
Total derivative liabilities		533		778
Total liabilities		533		778
Level 2 | Interest rate swaps
Assets
Derivative assets		538		609
Liabilities
Total derivative liabilities		379		645
Level 2 | Foreign exchange contracts
Assets
Derivative assets		148		134
Liabilities
Total derivative liabilities		149		126
Level 2 | Other contracts
Assets
Derivative assets		7		16
Liabilities
Total derivative liabilities		5		7
Level 2 | Mortgage-backed securities
Assets
Securities available for sale		20,139		19,446
Level 2 | State and political subdivisions
Assets
Securities available for sale		6		8
Level 2 | Equity securities
Assets
Securities available for sale				17
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		326		504
Level 2 | Commercial loans held for sale
Assets
Loans held for sale		171		79
Level 3
Assets
Securities available for sale		0		0
Loans held for sale		0		0
Derivative assets		0		0
Other investment securities, at fair value:
Money market mutual fund		0		0
Other investments		0		0
Total other investment securities, at fair value		0		0
Total assets		0		0
Liabilities
Total derivative liabilities		0		0
Total liabilities		0		0
Level 3 | Interest rate swaps
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 | Equity securities
Assets
Securities available for sale				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		0		0
Level 3 | Commercial loans held for sale
Assets
Loans held for sale		$ 0		$ 0

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

[2] Amounts reflect changes in the treatment of variation margin on certain centrally cleared derivatives.