Securities - Schedule of Amortized Cost and Fair Value of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,254	$ 3,242
Gross Unrealized Gains	5	17
Gross Unrealized Losses	(104)	(41)
Securities held to maturity, Fair Value	3,155	3,218
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	2
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Securities held to maturity, Fair Value	2
U.S. government-sponsored entities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	215	247
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(4)	(4)
Securities held to maturity, Fair Value	211	243
Agency mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,036	1,220
Gross Unrealized Gains	0	3
Gross Unrealized Losses	(26)	(9)
Securities held to maturity, Fair Value	1,010	1,214
Agency collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	794	777
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(24)	(20)
Securities held to maturity, Fair Value	771	757
Commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	126	80
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(1)	(1)
Securities held to maturity, Fair Value	126	80
States of the U.S. and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,080	917
Gross Unrealized Gains	3	13
Gross Unrealized Losses	(49)	(7)
Securities held to maturity, Fair Value	$ 1,034	$ 923