Securities - Summaries of Fair Values and Unrealized Losses of Securities, Segregated by Length of Impairment (Detail) $ in Millions	Dec. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	32	46
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	78	136
Debt securities, available-for-sale, number of positions \| Security	110	182
Debt securities, available-for-sale, less than 12 months	$ 685	$ 679
Debt securities, available-for-sale, 12 months or longer	712	1,988
Debt securities, available-for-sale, unrealized loss position	1,397	2,667
Debt securities, available-for-sale, less than 12 months, accumulated loss	(4)	(7)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	(6)	(58)
Debt securities, available-for-sale, accumulated loss	$ (10)	$ (65)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	90	373
Securities held to maturity, less than 12 months, fair value	$ 431	$ 952
Securities held to maturity, less than 12 months, unrealized losses	(4)	(31)
Securities held to maturity, greater than 12 months, fair value	648	1,720
Securities held to maturity, greater than 12 months, unrealized losses	(8)	(73)
Securities held to maturity, fair value, total	1,079	2,672
Securities held to maturity, unrealized losses, total	$ (12)	$ (104)
Less than 12 Months
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	30	210
12 Months or More
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	60	163
U.S. government agencies
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	5	20
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	15	0
Debt securities, available-for-sale, number of positions \| Security	20	20
Debt securities, available-for-sale, less than 12 months	$ 48	$ 145
Debt securities, available-for-sale, 12 months or longer	61	0
Debt securities, available-for-sale, unrealized loss position	109	145
Debt securities, available-for-sale, less than 12 months, accumulated loss	0	(1)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	(1)	0
Debt securities, available-for-sale, accumulated loss	$ (1)	$ (1)
U.S. government-sponsored entities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	0	1
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	6	11
Debt securities, available-for-sale, number of positions \| Security	6	12
Debt securities, available-for-sale, less than 12 months	$ 0	$ 36
Debt securities, available-for-sale, 12 months or longer	130	227
Debt securities, available-for-sale, unrealized loss position	130	263
Debt securities, available-for-sale, less than 12 months, accumulated loss	0	0
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0	(4)
Debt securities, available-for-sale, accumulated loss	$ 0	$ (4)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	8	12
Securities held to maturity, less than 12 months, fair value	$ 0	$ 0
Securities held to maturity, less than 12 months, unrealized losses	0	0
Securities held to maturity, greater than 12 months, fair value	160	211
Securities held to maturity, greater than 12 months, unrealized losses	0	(4)
Securities held to maturity, fair value, total	160	211
Securities held to maturity, unrealized losses, total	$ 0	$ (4)
U.S. government-sponsored entities \| Less than 12 Months
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	0	0
U.S. government-sponsored entities \| 12 Months or More
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	8	12
Agency mortgage-backed securities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	13	16
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	24	71
Debt securities, available-for-sale, number of positions \| Security	37	87
Debt securities, available-for-sale, less than 12 months	$ 200	$ 259
Debt securities, available-for-sale, 12 months or longer	314	1,159
Debt securities, available-for-sale, unrealized loss position	514	1,418
Debt securities, available-for-sale, less than 12 months, accumulated loss	(1)	(4)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	(2)	(32)
Debt securities, available-for-sale, accumulated loss	$ (3)	$ (36)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	16	90
Securities held to maturity, less than 12 months, fair value	$ 134	$ 294
Securities held to maturity, less than 12 months, unrealized losses	(1)	(4)
Securities held to maturity, greater than 12 months, fair value	135	694
Securities held to maturity, greater than 12 months, unrealized losses	(1)	(22)
Securities held to maturity, fair value, total	269	988
Securities held to maturity, unrealized losses, total	$ (2)	$ (26)
Agency mortgage-backed securities \| Less than 12 Months
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	5	43
Agency mortgage-backed securities \| 12 Months or More
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	11	47
Agency collateralized mortgage obligations
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	11	2
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	32	47
Debt securities, available-for-sale, number of positions \| Security	43	49
Debt securities, available-for-sale, less than 12 months	$ 323	$ 82
Debt securities, available-for-sale, 12 months or longer	205	590
Debt securities, available-for-sale, unrealized loss position	528	672
Debt securities, available-for-sale, less than 12 months, accumulated loss	(1)	(1)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	(3)	(22)
Debt securities, available-for-sale, accumulated loss	$ (4)	$ (23)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	35	52
Securities held to maturity, less than 12 months, fair value	$ 50	$ 42
Securities held to maturity, less than 12 months, unrealized losses	0	0
Securities held to maturity, greater than 12 months, fair value	317	611
Securities held to maturity, greater than 12 months, unrealized losses	(6)	(24)
Securities held to maturity, fair value, total	367	653
Securities held to maturity, unrealized losses, total	$ (6)	$ (24)
Agency collateralized mortgage obligations \| Less than 12 Months
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	2	3
Agency collateralized mortgage obligations \| 12 Months or More
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	33	49
Non-agency collateralized mortgage obligations
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security		1
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security		0
Debt securities, available-for-sale, number of positions \| Security		1
Debt securities, available-for-sale, less than 12 months		$ 0
Debt securities, available-for-sale, 12 months or longer		0
Debt securities, available-for-sale, unrealized loss position		0
Debt securities, available-for-sale, less than 12 months, accumulated loss		0
Debt securities, available-for-sale, 12 months or longer, accumulated loss		0
Debt securities, available-for-sale, accumulated loss		$ 0
Commercial mortgage-backed securities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	3	4
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	0	0
Debt securities, available-for-sale, number of positions \| Security	3	4
Debt securities, available-for-sale, less than 12 months	$ 114	$ 155
Debt securities, available-for-sale, 12 months or longer	0	0
Debt securities, available-for-sale, unrealized loss position	114	155
Debt securities, available-for-sale, less than 12 months, accumulated loss	(2)	(1)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0	0
Debt securities, available-for-sale, accumulated loss	$ (2)	$ (1)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	6	9
Securities held to maturity, less than 12 months, fair value	$ 184	$ 26
Securities held to maturity, less than 12 months, unrealized losses	(2)	0
Securities held to maturity, greater than 12 months, fair value	7	43
Securities held to maturity, greater than 12 months, unrealized losses	0	(1)
Securities held to maturity, fair value, total	191	69
Securities held to maturity, unrealized losses, total	$ (2)	$ (1)
Commercial mortgage-backed securities \| Less than 12 Months
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	5	5
Commercial mortgage-backed securities \| 12 Months or More
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	1	4
States of the U.S. and political subdivisions
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	0	2
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	0	6
Debt securities, available-for-sale, number of positions \| Security	0	8
Debt securities, available-for-sale, less than 12 months	$ 0	$ 2
Debt securities, available-for-sale, 12 months or longer	0	10
Debt securities, available-for-sale, unrealized loss position	0	12
Debt securities, available-for-sale, less than 12 months, accumulated loss	0	0
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0	0
Debt securities, available-for-sale, accumulated loss	$ 0	$ 0
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	25	210
Securities held to maturity, less than 12 months, fair value	$ 63	$ 590
Securities held to maturity, less than 12 months, unrealized losses	(1)	(27)
Securities held to maturity, greater than 12 months, fair value	29	161
Securities held to maturity, greater than 12 months, unrealized losses	(1)	(22)
Securities held to maturity, fair value, total	92	751
Securities held to maturity, unrealized losses, total	$ (2)	$ (49)
States of the U.S. and political subdivisions \| Less than 12 Months
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	18	159
States of the U.S. and political subdivisions \| 12 Months or More
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions \| Security	7	51
Other debt securities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	0	0
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	1	1
Debt securities, available-for-sale, number of positions \| Security	1	1
Debt securities, available-for-sale, less than 12 months	$ 0	$ 0
Debt securities, available-for-sale, 12 months or longer	2	2
Debt securities, available-for-sale, unrealized loss position	2	2
Debt securities, available-for-sale, less than 12 months, accumulated loss	0	0
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0	0
Debt securities, available-for-sale, accumulated loss	$ 0	$ 0

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

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