SECURITIES - Amortized Cost and Fair Value of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investment Securities Held To Maturity [Line Items]
Amortized Cost	$ 3,171	$ 3,254
Gross Unrealized Gains	15	5
Gross Unrealized Losses	(47)	(104)
Fair Value	3,139	3,155
U.S. Treasury [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	1	1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1	1
US Government Agencies [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	2	2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	2	2
U.S. Government-Sponsored Entities [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	200	215
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(4)
Fair Value	197	211
Agency Mortgage-Backed Securities [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	997	1,036
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(11)	(26)
Fair Value	987	1,010
Agency Collateralized Mortgage Obligations [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	764	794
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(16)	(24)
Fair Value	751	771
Commercial Mortgage-Backed Securities [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	104	126
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(1)	(1)
Fair Value	105	126
States of the U.S. and Political Subdivisions [Member]
Investment Securities Held To Maturity [Line Items]
Amortized Cost	1,103	1,080
Gross Unrealized Gains	9	3
Gross Unrealized Losses	(16)	(49)
Fair Value	$ 1,096	$ 1,034