SECURITIES - Schedule of Investments (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Securities Available-for-sale, Amortized Cost		$ 20,394	$ 19,713
Securities Available-for-sale, Gross Unrealized Gains		47	80
Securities Available-for-sale, Gross Unrealized Losses		(284)	(292)
Securities Available-for-sale, Fair Value	[1]	20,157	19,501
Securities Held-to-maturity, Amortized Cost		4,685	5,071
Securities Held-to-maturity, Gross Unrealized Gain		29	31
Securities Held-to-maturity, Gross Unrealized Losses		(46)	(44)
Securities held-to-maturity, Fair Value		4,668	5,058
Money market mutual fund, Amortized Cost		165	91
Money market mutual fund, Gross Unrealized Gains		0	0
Money market mutual fund, Gross Unrealized Losses		0	0
Money market mutual fund, Fair Value		165	91
Total other investment securities, at fair value, Amortized Cost		169	96
Total other investments securities, at fair value, Gross Unrealized Gains		0	0
Total other investments securities, at fair value, Gross Unrealized Losses		0	0
Total other investment securities, at fair value		169	96
Other Investment Securities, at Cost, Amortized Cost Basis		7	0
Other Investment Securities, at Cost, Accumulated Gross Unrealized Gain, before Tax		0	0
Other Investment Securities, at Cost, Accumulated Gross Unrealized Loss, before Tax		0	0
Other Investment Securities, at Cost		7	0
Total other investment securities, at cost		722	942
Total other investment securities, at cost, Gross Unrealized Gain		0	0
Total other investment securities, at cost, Gross Unrealized Losses		0	0
Total other investment securities, at cost, Fair Value		722	942
U.S. Treasury and other
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost		12	30
Debt Securities Available-for-sale, Gross Unrealized Gains		0	0
Debt Securities Available-for-sale, Gross Unrealized Losses		0	0
Debt Securities Available-for-sale, Fair Value		12	30
State and political subdivisions
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost		6	8
Debt Securities Available-for-sale, Gross Unrealized Gains		0	0
Debt Securities Available-for-sale, Gross Unrealized Losses		0	0
Debt Securities Available-for-sale, Fair Value		6	8
Federal agencies and U.S. government sponsored entities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost		20,065	19,231
Debt Securities Available-for-sale, Gross Unrealized Gains		40	78
Debt Securities Available-for-sale, Gross Unrealized Losses		(277)	(264)
Debt Securities Available-for-sale, Fair Value		19,828	19,045
Securities Held-to-maturity, Amortized Cost		3,853	4,126
Securities Held-to-maturity, Gross Unrealized Gain		7	12
Securities Held-to-maturity, Gross Unrealized Losses		(46)	(44)
Securities held-to-maturity, Fair Value		3,814	4,094
Other/non-agency
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost		311	427
Debt Securities Available-for-sale, Gross Unrealized Gains		7	2
Debt Securities Available-for-sale, Gross Unrealized Losses		(7)	(28)
Debt Securities Available-for-sale, Fair Value		311	401
Securities Held-to-maturity, Amortized Cost		832	945
Securities Held-to-maturity, Gross Unrealized Gain		22	19
Securities Held-to-maturity, Gross Unrealized Losses		0	0
Securities held-to-maturity, Fair Value		854	964
Total mortgage-backed securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost		20,376	19,658
Debt Securities Available-for-sale, Gross Unrealized Gains		47	80
Debt Securities Available-for-sale, Gross Unrealized Losses		(284)	(292)
Debt Securities Available-for-sale, Fair Value		20,139	19,446
Total debt securities available for sale
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost		20,394	19,696
Debt Securities Available-for-sale, Gross Unrealized Gains		47	80
Debt Securities Available-for-sale, Gross Unrealized Losses		(284)	(292)
Debt Securities Available-for-sale, Fair Value		20,157	19,484
Marketable equity securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Equity Securities Available-for-sale, Amortized Cost		0	5
Equity Securities Available-for-sale, Gross Unrealized Gains		0	0
Equity Securities Available-for-sale, Gross Unrealized Losses		0	0
Equity Securities Available-for-sale, Fair Value		0	5
Other equity securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Equity Securities Available-for-sale, Amortized Cost		0	12
Equity Securities Available-for-sale, Gross Unrealized Gains		0	0
Equity Securities Available-for-sale, Gross Unrealized Losses		0	0
Equity Securities Available-for-sale, Fair Value		0	12
Total equity securities available for sale
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Equity Securities Available-for-sale, Amortized Cost		0	17
Equity Securities Available-for-sale, Gross Unrealized Gains		0	0
Equity Securities Available-for-sale, Gross Unrealized Losses		0	0
Equity Securities Available-for-sale, Fair Value		0	17
Other investments
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Other investments, Amortized Cost		4	5
Other investments, Accumulated Gross Unrealized Gains		0	0
Other investments, Accumulated Gross Unrealized Losses		0	0
Other investments, Fair Value		4	5
Federal Reserve Bank stock
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Federal Reserve Bank stock, Amortized Cost		463	463
Federal Reserve Bank stock, Gross Unrealized Gains		0	0
Federal Reserve Bank stock, Gross Unrealized Losses		0	0
Federal Reserve Bank stock, Fair Value		463	463
Federal Home Loan Bank stock
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Federal Home Loan Bank stock, Amortized Cost		252	479
Federal Home Loan Bank stock, Gross Unrealized Gain		0	0
Federal Home Loan Bank stock, Gross Unrealized Losses		0	0
Federal Home Loan Bank stock, Fair Value		$ 252	$ 479

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