SECURITIES - Fair Values and Unrealized Losses of Impaired Securities, by Length of Impairment (Detail) $ in Millions	Sep. 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	28	46
Number of available for sale securities, Greater than 1 year \| Security	80	136
Number of available for sale securities \| Security	108	182
Securities available for sale, less than 12 months, fair value	$ 496	$ 679
Securities available for sale, 12 months or more, fair value	783	1,988
Securities available for sale, fair value, total	1,279	2,667
Securities available for sale, less than 12 months, unrealized losses	(2)	(7)
Securities available for sale, 12 months or more, unrealized losses	(8)	(58)
Securities available for sale, unrealized losses, total	$ (10)	$ (65)
Number of securities held to maturity positions \| Security	84	373
Securities held to maturity, less than 12 months, fair value	$ 287	$ 952
Securities held to maturity, greater than 12 months, fair value	716	1,720
Securities held to maturity, fair value, total	1,003	2,672
Securities held to maturity, less than 12 months, unrealized losses	(1)	(31)
Securities held to maturity, greater than 12 months, unrealized losses	(9)	(73)
Securities held to maturity, unrealized losses, total	$ (10)	$ (104)
Less Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	19	210
Greater Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	65	163
US Government Agencies [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	6	20
Number of available for sale securities, Greater than 1 year \| Security	13	0
Number of available for sale securities \| Security	19	20
Securities available for sale, less than 12 months, fair value	$ 48	$ 145
Securities available for sale, 12 months or more, fair value	59	0
Securities available for sale, fair value, total	107	145
Securities available for sale, less than 12 months, unrealized losses	0	(1)
Securities available for sale, 12 months or more, unrealized losses	(1)	0
Securities available for sale, unrealized losses, total	$ (1)	$ (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	7	11
Number of available for sale securities \| Security	7	12
Securities available for sale, less than 12 months, fair value	$ 0	$ 36
Securities available for sale, 12 months or more, fair value	149	227
Securities available for sale, fair value, total	149	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	17	16
Number of available for sale securities, Greater than 1 year \| Security	25	71
Number of available for sale securities \| Security	42	87
Securities available for sale, less than 12 months, fair value	$ 281	$ 259
Securities available for sale, 12 months or more, fair value	343	1,159
Securities available for sale, fair value, total	624	1,418
Securities available for sale, less than 12 months, unrealized losses	(1)	(4)
Securities available for sale, 12 months or more, unrealized losses	(3)	(32)
Securities available for sale, unrealized losses, total	$ (4)	$ (36)
Number of securities held to maturity positions \| Security	17	90
Securities held to maturity, less than 12 months, fair value	$ 163	$ 294
Securities held to maturity, greater than 12 months, fair value	143	694
Securities held to maturity, fair value, total	306	988
Securities held to maturity, less than 12 months, unrealized losses	(1)	(4)
Securities held to maturity, greater than 12 months, unrealized losses	(2)	(22)
Securities held to maturity, unrealized losses, total	$ (3)	$ (26)
Agency Mortgage-Backed Securities [Member] \| Less Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	6	43
Agency Mortgage-Backed Securities [Member] \| Greater Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	11	47
Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year \| Security	4	2
Number of available for sale securities, Greater than 1 year \| Security	33	47
Number of available for sale securities \| Security	37	49
Securities available for sale, less than 12 months, fair value	$ 113	$ 82
Securities available for sale, 12 months or more, fair value	229	590
Securities available for sale, fair value, total	342	672
Securities available for sale, less than 12 months, unrealized losses	0	(1)
Securities available for sale, 12 months or more, unrealized losses	(3)	(22)
Securities available for sale, unrealized losses, total	$ (3)	$ (23)
Number of securities held to maturity positions \| Security	36	52
Securities held to maturity, less than 12 months, fair value	$ 11	$ 42
Securities held to maturity, greater than 12 months, fair value	345	611
Securities held to maturity, fair value, total	356	653
Securities held to maturity, less than 12 months, unrealized losses	0	0
Securities held to maturity, greater than 12 months, unrealized losses	(5)	(24)
Securities held to maturity, unrealized losses, total	$ (5)	$ (24)
Agency Collateralized Mortgage Obligations [Member] \| Less Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	1	3
Agency Collateralized Mortgage Obligations [Member] \| Greater Than Twelve Months [Member]
Schedule Of Securities [Line Items]
Number of securities held to maturity positions \| Security	35	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	1	4
Number of available for sale securities, Greater than 1 year \| Security	0	0
Number of available for sale securities \| Security	1	4
Securities available for sale, less than 12 months, fair value	$ 54	$ 155
Securities available for sale, 12 months or more, fair value	0	0
Securities available for sale, fair value, total	54	155
Securities available for sale, less than 12 months, unrealized losses	(1)	(1)
Securities available for sale, 12 months or more, unrealized losses	0	0
Securities available for sale, unrealized losses, total	$ (1)	$ (1)
Number of securities held to maturity positions \| Security	4	9
Securities held to maturity, less than 12 months, fair value	$ 77	$ 26
Securities held to maturity, greater than 12 months, fair value	9	43
Securities held to maturity, fair value, total	86	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	2	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	1	6
Number of available for sale securities \| Security	1	8
Securities available for sale, less than 12 months, fair value	$ 0	$ 2
Securities available for sale, 12 months or more, fair value	1	10
Securities available for sale, fair value, total	1	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	17	210
Securities held to maturity, less than 12 months, fair value	$ 36	$ 590
Securities held to maturity, greater than 12 months, fair value	30	161
Securities held to maturity, fair value, total	66	751
Securities held to maturity, less than 12 months, unrealized losses	0	(27)
Securities held to maturity, greater than 12 months, unrealized losses	(1)	(22)
Securities held to maturity, unrealized losses, total	$ (1)	$ (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	10	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	7	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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