Securities - Summaries of Fair Values and Unrealized Losses of Securities, Segregated by Length of Impairment (Detail) $ in Millions	Dec. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions | Security	46	71
Debt securities, available-for-sale, 12 months or longer, number of positions | Security	136	75
Debt securities, available-for-sale, number of positions | Security	182	146
Debt securities, available-for-sale, less than 12 months	$ 679	$ 1,504
Debt securities, available-for-sale, 12 months or longer	1,988	1,016
Debt securities, available-for-sale, unrealized loss position	2,667	2,520
Debt securities, available-for-sale, less than 12 months, accumulated loss	(7)	(14)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	(58)	(26)
Debt securities, available-for-sale, accumulated loss	$ (65)	$ (40)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	373	168
Securities held to maturity, less than 12 months, fair value	$ 952	$ 1,062
Securities held to maturity, less than 12 months, unrealized losses	(31)	(8)
Securities held to maturity, greater than 12 months, fair value	1,720	984
Securities held to maturity, greater than 12 months, unrealized losses	(73)	(33)
Securities held to maturity, fair value, total	2,672	2,046
Securities held to maturity, unrealized losses, total	$ (104)	$ (41)
U.S. government agencies
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions | Security	20
Debt securities, available-for-sale, 12 months or longer, number of positions | Security	0
Debt securities, available-for-sale, number of positions | Security	20
Debt securities, available-for-sale, less than 12 months	$ 145
Debt securities, available-for-sale, 12 months or longer	0
Debt securities, available-for-sale, unrealized loss position	145
Debt securities, available-for-sale, less than 12 months, accumulated loss	(1)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0
Debt securities, available-for-sale, accumulated loss	$ (1)
U.S. government-sponsored entities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions | Security	1	7
Debt securities, available-for-sale, 12 months or longer, number of positions | Security	11	10
Debt securities, available-for-sale, number of positions | Security	12	17
Debt securities, available-for-sale, less than 12 months	$ 36	$ 107
Debt securities, available-for-sale, 12 months or longer	227	201
Debt securities, available-for-sale, unrealized loss position	263	308
Debt securities, available-for-sale, less than 12 months, accumulated loss	0	0
Debt securities, available-for-sale, 12 months or longer, accumulated loss	(4)	(4)
Debt securities, available-for-sale, accumulated loss	$ (4)	$ (4)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	12	14
Securities held to maturity, less than 12 months, fair value	$ 0	$ 55
Securities held to maturity, less than 12 months, unrealized losses	0	0
Securities held to maturity, greater than 12 months, fair value	211	186
Securities held to maturity, greater than 12 months, unrealized losses	(4)	(4)
Securities held to maturity, fair value, total	211	241
Securities held to maturity, unrealized losses, total	$ (4)	$ (4)
Agency mortgage-backed securities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions | Security	16	43
Debt securities, available-for-sale, 12 months or longer, number of positions | Security	71	28
Debt securities, available-for-sale, number of positions | Security	87	71
Debt securities, available-for-sale, less than 12 months	$ 259	$ 977
Debt securities, available-for-sale, 12 months or longer	1,159	473
Debt securities, available-for-sale, unrealized loss position	1,418	1,450
Debt securities, available-for-sale, less than 12 months, accumulated loss	(4)	(8)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	(32)	(10)
Debt securities, available-for-sale, accumulated loss	$ (36)	$ (18)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	90	47
Securities held to maturity, less than 12 months, fair value	$ 294	$ 648
Securities held to maturity, less than 12 months, unrealized losses	(4)	(5)
Securities held to maturity, greater than 12 months, fair value	694	184
Securities held to maturity, greater than 12 months, unrealized losses	(22)	(4)
Securities held to maturity, fair value, total	988	832
Securities held to maturity, unrealized losses, total	$ (26)	$ (9)
Agency collateralized mortgage obligations
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions | Security	2	14
Debt securities, available-for-sale, 12 months or longer, number of positions | Security	47	33
Debt securities, available-for-sale, number of positions | Security	49	47
Debt securities, available-for-sale, less than 12 months	$ 82	$ 409
Debt securities, available-for-sale, 12 months or longer	590	336
Debt securities, available-for-sale, unrealized loss position	672	745
Debt securities, available-for-sale, less than 12 months, accumulated loss	(1)	(6)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	(22)	(12)
Debt securities, available-for-sale, accumulated loss	$ (23)	$ (18)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	52	49
Securities held to maturity, less than 12 months, fair value	$ 42	$ 276
Securities held to maturity, less than 12 months, unrealized losses	0	(2)
Securities held to maturity, greater than 12 months, fair value	611	473
Securities held to maturity, greater than 12 months, unrealized losses	(24)	(18)
Securities held to maturity, fair value, total	653	749
Securities held to maturity, unrealized losses, total	$ (24)	$ (20)
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	4
Debt securities, available-for-sale, 12 months or longer, number of positions | Security	0
Debt securities, available-for-sale, number of positions | Security	4
Debt securities, available-for-sale, less than 12 months	$ 155
Debt securities, available-for-sale, 12 months or longer	0
Debt securities, available-for-sale, unrealized loss position	155
Debt securities, available-for-sale, less than 12 months, accumulated loss	(1)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0
Debt securities, available-for-sale, accumulated loss	$ (1)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	9	5
Securities held to maturity, less than 12 months, fair value	$ 26	$ 26
Securities held to maturity, less than 12 months, unrealized losses	0	0
Securities held to maturity, greater than 12 months, fair value	43	20
Securities held to maturity, greater than 12 months, unrealized losses	(1)	(1)
Securities held to maturity, fair value, total	69	46
Securities held to maturity, unrealized losses, total	$ (1)	$ (1)
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	2	7
Debt securities, available-for-sale, 12 months or longer, number of positions | Security	6	1
Debt securities, available-for-sale, number of positions | Security	8	8
Debt securities, available-for-sale, less than 12 months	$ 2	$ 11
Debt securities, available-for-sale, 12 months or longer	10	1
Debt securities, available-for-sale, unrealized loss position	12	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	210	53
Securities held to maturity, less than 12 months, fair value	$ 590	$ 57
Securities held to maturity, less than 12 months, unrealized losses	(27)	(1)
Securities held to maturity, greater than 12 months, fair value	161	121
Securities held to maturity, greater than 12 months, unrealized losses	(22)	(6)
Securities held to maturity, fair value, total	751	178
Securities held to maturity, unrealized losses, total	$ (49)	$ (7)
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	3
Debt securities, available-for-sale, number of positions | Security	1	3
Debt securities, available-for-sale, less than 12 months	$ 0	$ 0
Debt securities, available-for-sale, 12 months or longer	2	5
Debt securities, available-for-sale, unrealized loss position	2	5
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
Less than 12 Months
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	210	73
Less than 12 Months | U.S. government-sponsored entities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	0	4
Less than 12 Months | Agency mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	43	36
Less than 12 Months | Agency collateralized mortgage obligations
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	3	14
Less than 12 Months | Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	5	3
Less than 12 Months | States of the U.S. and political subdivisions
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	159	16
12 Months or More
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	163	95
12 Months or More | U.S. government-sponsored entities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	12	10
12 Months or More | Agency mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	47	11
12 Months or More | Agency collateralized mortgage obligations
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	49	35
12 Months or More | Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	4	2
12 Months or More | States of the U.S. and political subdivisions
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Number of positions | Security	51	37