SECURITIES - Schedule of Investments (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Securities Available-for-sale, Amortized Cost	$ 18,517	$ 18,433
Securities Available-for-sale, Gross Unrealized Gains	231	308
Securities Available-for-sale, Gross Unrealized Losses	(86)	(85)
Securities Available-for-sale, Fair Value	18,662	18,656
Securities Held-to-maturity, Amortized Cost	5,567	5,148
Securities Held-to-maturity, Gross Unrealized Gain	56	76
Securities Held-to-maturity, Gross Unrealized Losses	(12)	(31)
Securities held-to-maturity, Fair Value	5,611	5,193
Other Investment Securities, Amortized Cost	866	872
Other Investment Securities, Gross Unrealized Gains	0	0
Other Investment Securities, Gross Unrealized Losses	0	0
Other Investment Securities, Fair Value	866	872
U.S. Treasury
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	30	15
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	30	15
State and political subdivisions
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	9	10
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	9	10
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	17,830	17,683
Debt Securities Available-for-sale, Gross Unrealized Gains	227	301
Debt Securities Available-for-sale, Gross Unrealized Losses	(56)	(50)
Debt Securities Available-for-sale, Fair Value	18,001	17,934
Securities Held-to-maturity, Amortized Cost	4,253	3,728
Securities Held-to-maturity, Gross Unrealized Gain	28	22
Securities Held-to-maturity, Gross Unrealized Losses	(12)	(31)
Securities held-to-maturity, Fair Value	4,269	3,719
Other/non-agency
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	631	703
Debt Securities Available-for-sale, Gross Unrealized Gains	4	4
Debt Securities Available-for-sale, Gross Unrealized Losses	(30)	(35)
Debt Securities Available-for-sale, Fair Value	605	672
Securities Held-to-maturity, Amortized Cost	1,314	1,420
Securities Held-to-maturity, Gross Unrealized Gain	28	54
Securities Held-to-maturity, Gross Unrealized Losses	0	0
Securities held-to-maturity, Fair Value	1,342	1,474
Total mortgage-backed securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	18,461	18,386
Debt Securities Available-for-sale, Gross Unrealized Gains	231	305
Debt Securities Available-for-sale, Gross Unrealized Losses	(86)	(85)
Debt Securities Available-for-sale, Fair Value	18,606	18,606
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	18,500	18,411
Debt Securities Available-for-sale, Gross Unrealized Gains	231	305
Debt Securities Available-for-sale, Gross Unrealized Losses	(86)	(85)
Debt Securities Available-for-sale, Fair Value	18,645	18,631
Marketable equity securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Equity Securities Available-for-sale, Amortized Cost	5	10
Equity Securities Available-for-sale, Gross Unrealized Gains	0	3
Equity Securities Available-for-sale, Gross Unrealized Losses	0	0
Equity Securities Available-for-sale, Fair Value	5	13
Other equity securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Equity Securities Available-for-sale, Amortized Cost	12	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	12	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	17	22
Equity Securities Available-for-sale, Gross Unrealized Gains	0	3
Equity Securities Available-for-sale, Gross Unrealized Losses	0	0
Equity Securities Available-for-sale, Fair Value	17	25
Federal Reserve Bank stock
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Federal Reserve Bank Stock, Amortized Cost	468	477
Federal Reserve Bank Stock, Gross Unrealized Gains	0	0
Federal Reserve Bank Stock, Gross Unrealized Loss	0	0
Federal Reserve Bank Stock, Fair Value	468	477
Federal Home Loan Bank stock
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Federal Home Loan Bank Stock, Amortized Cost	393	390
Federal Home Loan Bank Stock, Gross Unrealized Gain	0	0
Federal Home Loan Bank Stock, Gross Unrealized Loss	0	0
Federal Home Loan Bank Stock, Fair Value	393	390
Total other investment securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Venture Capital, Amortized Cost	5	5
Venture Capital, Gross Unrealized Gain	0	0
Venture Capital, Gross Unrealized Loss	0	0
Venture Capital, Fair Value	$ 5	$ 5