Securities - Schedule of Amortized Cost and Fair Value of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,275	$ 3,254
Gross Unrealized Gains	42	5
Gross Unrealized Losses	(12)	(104)
Securities held to maturity, Fair Value	3,305	3,155
U.S. Treasury
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1	1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Securities held to maturity, Fair Value	1	1
U.S. government agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1	2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Securities held to maturity, Fair Value	1	2
U.S. government-sponsored entities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	175	215
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(4)
Securities held to maturity, Fair Value	175	211
Agency mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	949	1,036
Gross Unrealized Gains	8	0
Gross Unrealized Losses	(2)	(26)
Securities held to maturity, Fair Value	955	1,010
Agency collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	721	794
Gross Unrealized Gains	5	1
Gross Unrealized Losses	(6)	(24)
Securities held to maturity, Fair Value	720	771
Commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	308	126
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(2)	(1)
Securities held to maturity, Fair Value	309	126
States of the U.S. and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,120	1,080
Gross Unrealized Gains	26	3
Gross Unrealized Losses	(2)	(49)
Securities held to maturity, Fair Value	$ 1,144	$ 1,034