Securities - Schedule of Amortized Cost and Fair Value of Securities Available for Sale (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Amortized Cost	$ 3,380	$ 3,275
Gross Unrealized Gains	84	24
Gross Unrealized Losses	(1)	(10)
Fair Value	3,463	3,289
U.S. Treasury
Schedule of Investments [Line Items]
Amortized Cost	600
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	600
U.S. government agencies
Schedule of Investments [Line Items]
Amortized Cost	172	152
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Fair Value	172	151
U.S. government-sponsored entities
Schedule of Investments [Line Items]
Amortized Cost	160	225
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Fair Value	161	226
Agency mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost	959	1,310
Gross Unrealized Gains	35	7
Gross Unrealized Losses	0	(3)
Fair Value	994	1,314
Agency collateralized mortgage obligations
Schedule of Investments [Line Items]
Amortized Cost	1,094	1,234
Gross Unrealized Gains	31	10
Gross Unrealized Losses	(1)	(4)
Fair Value	1,124	1,240
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost	361	341
Gross Unrealized Gains	17	6
Gross Unrealized Losses	0	(2)
Fair Value	378	345
States of the U.S. and political subdivisions (municipals)
Schedule of Investments [Line Items]
Amortized Cost	32	11
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	32	11
Other debt securities
Schedule of Investments [Line Items]
Amortized Cost	2	2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 2	$ 2