Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Sep. 30, 2021 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 78,900
Unrealized Losses Less Than 12 Months	(1,632)
Fair Value 12 Months or Greater	2,479
Unrealized Losses 12 Months or Greater	(98)
Fair Value Total	81,379
Unrealized Losses Total	(1,730)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	6,023
Unrealized Losses Less Than 12 Months	(163)
Fair Value 12 Months or Greater	980
Unrealized Losses 12 Months or Greater	(36)
Fair Value Total	7,003
Unrealized Losses Total	(199)
Residential Mortgage-Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	66,781
Unrealized Losses Less Than 12 Months	(1,328)
Fair Value 12 Months or Greater	375
Unrealized Losses 12 Months or Greater	(9)
Fair Value Total	67,156
Unrealized Losses Total	(1,337)
Commercial Mortgage Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	3,947
Unrealized Losses Less Than 12 Months	(88)
Fair Value 12 Months or Greater	1,122
Unrealized Losses 12 Months or Greater	(53)
Fair Value Total	5,069
Unrealized Losses Total	(141)
Asset-backed Securities [member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	2
Fair Value Total	2
Obligations of State and Political Subdivisions [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	2,145
Unrealized Losses Less Than 12 Months	(53)
Fair Value Total	2,145
Unrealized Losses Total	(53)
Other [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	4
Fair Value Total	$ 4