SECURITIES - Schedule of Investments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Securities Held-to-maturity, Amortized Cost	$ 4,417	$ 4,685
Securities Held-to-maturity, Gross Unrealized Gain	5	29
Securities Held-to-maturity, Gross Unrealized Losses	(162)	(46)
Securities held-to-maturity, Fair Value	4,260	4,668
Money market mutual fund, Amortized Cost	170	165
Money Market Mutual Fund, Gross Unrealized Gain	0	0
Money Market Mutual Fund, Gross Unrealized Loss	0	0
Money market mutual fund, Fair Value	170	165
Total other investment securities, at fair value, Amortized Cost	170	169
Total other investments securities, at fair value, at cost, Gross Unrealized Gains	0	0
Total other investments securities, at fair value, Gross Unrealized Losses	0	0
Total other investment securities, at fair value, Fair Value	170	169
Other Equity Securities, at Cost, Amortized Cost	7	7
Other Equity Securities, at Cost, Gross Unrealized Gain	0	0
Other Equity Securities, at Cost, Gross Unrealized Loss	0	0
Other Equity Securities, at Cost, Fair Value	7	7
Total equity securities, at cost	769	722
Total equity securities, at cost, Gross Unrealized Gain	0	0
Total equity securities, at cost, Gross Unrealized Losses	0	0
Total equity securities, at cost, Fair Value	769	722
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	12
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	12
State and political subdivisions
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	6	6
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	6
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	20,559	20,065
Debt Securities Available-for-sale, Gross Unrealized Gains	17	40
Debt Securities Available-for-sale, Gross Unrealized Losses	(705)	(277)
Debt Securities Available-for-sale, Fair Value	19,871	19,828
Securities Held-to-maturity, Amortized Cost	3,632	3,853
Securities Held-to-maturity, Gross Unrealized Gain	0	7
Securities Held-to-maturity, Gross Unrealized Losses	(159)	(46)
Securities held-to-maturity, Fair Value	3,473	3,814
Other/non-agency
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	269	311
Debt Securities Available-for-sale, Gross Unrealized Gains	5	7
Debt Securities Available-for-sale, Gross Unrealized Losses	(6)	(7)
Debt Securities Available-for-sale, Fair Value	268	311
Securities Held-to-maturity, Amortized Cost	785	832
Securities Held-to-maturity, Gross Unrealized Gain	5	22
Securities Held-to-maturity, Gross Unrealized Losses	(3)	0
Securities held-to-maturity, Fair Value	787	854
Total mortgage-backed securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	20,828	20,376
Debt Securities Available-for-sale, Gross Unrealized Gains	22	47
Debt Securities Available-for-sale, Gross Unrealized Losses	(711)	(284)
Debt Securities Available-for-sale, Fair Value	20,139	20,139
Securities Held-to-maturity, Amortized Cost	4,417	4,685
Securities Held-to-maturity, Gross Unrealized Gain	5	29
Securities Held-to-maturity, Gross Unrealized Losses	(162)	(46)
Securities held-to-maturity, Fair Value	4,260	4,668
Total debt securities available for sale, at fair value
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	20,846	20,394
Debt Securities Available-for-sale, Gross Unrealized Gains	22	47
Debt Securities Available-for-sale, Gross Unrealized Losses	(711)	(284)
Debt Securities Available-for-sale, Fair Value	20,157	20,157
Other investments
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Other Investments, Amortized Cost	0	4
Other Investments, Gross Unrealized Gain	0	0
Other Investments, Gross Unrealized Loss	0	0
Other Investments, Fair Value	0	4
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	299	252
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	$ 299	$ 252