SECURITIES - Fair Values and Unrealized Losses of Impaired Securities, by Length of Impairment (Detail) $ in Millions	Jun. 30, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	5	46
Number of available for sale securities, Greater than 1 year \| Security	116	136
Number of available for sale securities \| Security	121	182
Securities available for sale, less than 12 months, fair value	$ 19	$ 679
Securities available for sale, 12 months or more, fair value	1,505	1,988
Securities available for sale, fair value, total	1,524	2,667
Securities available for sale, less than 12 months, unrealized losses	0	(7)
Securities available for sale, 12 months or more, unrealized losses	(12)	(58)
Securities available for sale, unrealized losses, total	$ (12)	$ (65)
Number of securities held to maturity positions \| Security	130	373
Securities held to maturity, less than 12 months, fair value	$ 4	$ 952
Securities held to maturity, greater than 12 months, fair value	1,256	1,720
Securities held to maturity, fair value, total	1,260	2,672
Securities held to maturity, less than 12 months, unrealized losses	0	(31)
Securities held to maturity, greater than 12 months, unrealized losses	(18)	(73)
Securities held to maturity, unrealized losses, total	$ (18)	$ (104)
Less Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	1	210
Greater Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	129	163
US Government Agencies [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	5	20
Number of available for sale securities, Greater than 1 year \| Security	12	0
Number of available for sale securities \| Security	17	20
Securities available for sale, less than 12 months, fair value	$ 19	$ 145
Securities available for sale, 12 months or more, fair value	74	0
Securities available for sale, fair value, total	93	145
Securities available for sale, less than 12 months, unrealized losses	0	(1)
Securities available for sale, 12 months or more, unrealized losses	0	0
Securities available for sale, unrealized losses, total	$ 0	$ (1)
U.S. Government-Sponsored Entities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	0	1
Number of available for sale securities, Greater than 1 year \| Security	9	11
Number of available for sale securities \| Security	9	12
Securities available for sale, less than 12 months, fair value	$ 0	$ 36
Securities available for sale, 12 months or more, fair value	205	227
Securities available for sale, fair value, total	205	263
Securities available for sale, less than 12 months, unrealized losses	0	0
Securities available for sale, 12 months or more, unrealized losses	(1)	(4)
Securities available for sale, unrealized losses, total	$ (1)	$ (4)
Number of securities held to maturity positions \| Security	10	12
Securities held to maturity, less than 12 months, fair value	$ 0	$ 0
Securities held to maturity, greater than 12 months, fair value	189	211
Securities held to maturity, fair value, total	189	211
Securities held to maturity, less than 12 months, unrealized losses	0	0
Securities held to maturity, greater than 12 months, unrealized losses	(1)	(4)
Securities held to maturity, unrealized losses, total	$ (1)	$ (4)
U.S. Government-Sponsored Entities [Member] \| Less Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	0	0
U.S. Government-Sponsored Entities [Member] \| Greater Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	10	12
Agency Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	0	16
Number of available for sale securities, Greater than 1 year \| Security	53	71
Number of available for sale securities \| Security	53	87
Securities available for sale, less than 12 months, fair value	$ 0	$ 259
Securities available for sale, 12 months or more, fair value	849	1,159
Securities available for sale, fair value, total	849	1,418
Securities available for sale, less than 12 months, unrealized losses	0	(4)
Securities available for sale, 12 months or more, unrealized losses	(6)	(32)
Securities available for sale, unrealized losses, total	$ (6)	$ (36)
Number of securities held to maturity positions \| Security	36	90
Securities held to maturity, less than 12 months, fair value	$ 0	$ 294
Securities held to maturity, greater than 12 months, fair value	506	694
Securities held to maturity, fair value, total	506	988
Securities held to maturity, less than 12 months, unrealized losses	0	(4)
Securities held to maturity, greater than 12 months, unrealized losses	(4)	(22)
Securities held to maturity, unrealized losses, total	$ (4)	$ (26)
Agency Mortgage-Backed Securities [Member] \| Less Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	0	43
Agency Mortgage-Backed Securities [Member] \| Greater Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	36	47
Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	0	2
Number of available for sale securities, Greater than 1 year \| Security	38	47
Number of available for sale securities \| Security	38	49
Securities available for sale, less than 12 months, fair value	$ 0	$ 82
Securities available for sale, 12 months or more, fair value	370	590
Securities available for sale, fair value, total	370	672
Securities available for sale, less than 12 months, unrealized losses	0	(1)
Securities available for sale, 12 months or more, unrealized losses	(5)	(22)
Securities available for sale, unrealized losses, total	$ (5)	$ (23)
Number of securities held to maturity positions \| Security	38	52
Securities held to maturity, less than 12 months, fair value	$ 0	$ 42
Securities held to maturity, greater than 12 months, fair value	381	611
Securities held to maturity, fair value, total	381	653
Securities held to maturity, less than 12 months, unrealized losses	0	0
Securities held to maturity, greater than 12 months, unrealized losses	(8)	(24)
Securities held to maturity, unrealized losses, total	$ (8)	$ (24)
Agency Collateralized Mortgage Obligations [Member] \| Less Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	0	3
Agency Collateralized Mortgage Obligations [Member] \| Greater Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	38	49
Non Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security		1
Number of available for sale securities, Greater than 1 year \| Security		0
Number of available for sale securities \| Security		1
Securities available for sale, less than 12 months, fair value		$ 0
Securities available for sale, 12 months or more, fair value		0
Securities available for sale, fair value, total		0
Securities available for sale, less than 12 months, unrealized losses		0
Securities available for sale, 12 months or more, unrealized losses		0
Securities available for sale, unrealized losses, total		$ 0
Commercial Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security		4
Number of available for sale securities, Greater than 1 year \| Security		0
Number of available for sale securities \| Security		4
Securities available for sale, less than 12 months, fair value		$ 155
Securities available for sale, 12 months or more, fair value		0
Securities available for sale, fair value, total		155
Securities available for sale, less than 12 months, unrealized losses		(1)
Securities available for sale, 12 months or more, unrealized losses		0
Securities available for sale, unrealized losses, total		$ (1)
Number of securities held to maturity positions \| Security	2	9
Securities held to maturity, less than 12 months, fair value	$ 0	$ 26
Securities held to maturity, greater than 12 months, fair value	9	43
Securities held to maturity, fair value, total	9	69
Securities held to maturity, less than 12 months, unrealized losses	0	0
Securities held to maturity, greater than 12 months, unrealized losses	0	(1)
Securities held to maturity, unrealized losses, total	$ 0	$ (1)
Commercial Mortgage-Backed Securities [Member] \| Less Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	0	5
Commercial Mortgage-Backed Securities [Member] \| Greater Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	2	4
States of the U.S. and Political Subdivisions [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	0	2
Number of available for sale securities, Greater than 1 year \| Security	3	6
Number of available for sale securities \| Security	3	8
Securities available for sale, less than 12 months, fair value	$ 0	$ 2
Securities available for sale, 12 months or more, fair value	5	10
Securities available for sale, fair value, total	5	12
Securities available for sale, less than 12 months, unrealized losses	0	0
Securities available for sale, 12 months or more, unrealized losses	0	0
Securities available for sale, unrealized losses, total	$ 0	$ 0
Number of securities held to maturity positions \| Security	44	210
Securities held to maturity, less than 12 months, fair value	$ 4	$ 590
Securities held to maturity, greater than 12 months, fair value	171	161
Securities held to maturity, fair value, total	175	751
Securities held to maturity, less than 12 months, unrealized losses	0	(27)
Securities held to maturity, greater than 12 months, unrealized losses	(5)	(22)
Securities held to maturity, unrealized losses, total	$ (5)	$ (49)
States of the U.S. and Political Subdivisions [Member] \| Less Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	1	159
States of the U.S. and Political Subdivisions [Member] \| Greater Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	43	51
Other Debt Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	0	0
Number of available for sale securities, Greater than 1 year \| Security	1	1
Number of available for sale securities \| Security	1	1
Securities available for sale, less than 12 months, fair value	$ 0	$ 0
Securities available for sale, 12 months or more, fair value	2	2
Securities available for sale, fair value, total	2	2
Securities available for sale, less than 12 months, unrealized losses	0	0
Securities available for sale, 12 months or more, unrealized losses	0	0
Securities available for sale, unrealized losses, total	$ 0	$ 0

Debt Securities, Held-to-maturity, Unrealized Loss Positions, Qualitative Disclosure, Number of Positions

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