Investment Securities - Gross Unrealized Losses Excluding Portfolio Level Basis Adjustments and Fair Value of Company's Investment Securities (Detail) $ in Millions	Sep. 30, 2023 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 11,505
Unrealized Losses Less Than 12 Months	(257)
Fair Value 12 Months or Greater	52,900
Unrealized Losses 12 Months or Greater	(9,406)
Fair Value Total	64,405
Unrealized Losses Total	(9,663)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	672
Unrealized Losses Less Than 12 Months	(8)
Fair Value 12 Months or Greater	16,568
Unrealized Losses 12 Months or Greater	(2,571)
Fair Value Total	17,240
Unrealized Losses Total	(2,579)
Residential Mortgage-Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	3,085
Unrealized Losses Less Than 12 Months	(56)
Fair Value 12 Months or Greater	22,385
Unrealized Losses 12 Months or Greater	(3,223)
Fair Value Total	25,470
Unrealized Losses Total	(3,279)
Asset-Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	5,929
Unrealized Losses Less Than 12 Months	(63)
Fair Value Total	5,929
Unrealized Losses Total	(63)
Obligations of State and Political Subdivisions [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	1,813
Unrealized Losses Less Than 12 Months	(129)
Fair Value 12 Months or Greater	7,001
Unrealized Losses 12 Months or Greater	(1,843)
Fair Value Total	8,814
Unrealized Losses Total	(1,972)
Other [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	4
Fair Value Total	4
Agency Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	6,942
Unrealized Losses 12 Months or Greater	(1,769)
Fair Value Total	6,942
Unrealized Losses Total	(1,769)
Non Agency Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	6
Unrealized Losses Less Than 12 Months	(1)
Fair Value Total	6
Unrealized Losses Total	$ (1)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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