Securities - Schedule of Amortized Cost and Fair Value of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity, total	$ 2,868	$ 3,275
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	105	42
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(12)
Debt securities held-to-maturity, fair value	2,973	3,305
U.S. Treasury
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity, total	1	1
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt securities held-to-maturity, fair value	1	1
U.S. government agencies
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity, total	1	1
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt securities held-to-maturity, fair value	1	1
U.S. government-sponsored entities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity, total	120	175
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	1	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt securities held-to-maturity, fair value	121	175
Agency mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity, total	769	949
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	29	8
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(2)
Debt securities held-to-maturity, fair value	798	955
Agency collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity, total	562	721
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	17	5
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(6)
Debt securities held-to-maturity, fair value	579	720
Commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity, total	307	308
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	10	3
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(2)
Debt securities held-to-maturity, fair value	317	309
States of the U.S. and political subdivisions (municipals)
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity, total	1,108	1,120
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	48	26
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(2)
Debt securities held-to-maturity, fair value	$ 1,156	$ 1,144