Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Mar. 31, 2021 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 74,602
Unrealized Losses Less Than 12 Months	(2,289)
Fair Value 12 Months or Greater	158
Unrealized Losses 12 Months or Greater	(5)
Fair Value Total	74,760
Unrealized Losses Total	(2,294)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	9,519
Unrealized Losses Less Than 12 Months	(372)
Fair Value Total	9,519
Unrealized Losses Total	(372)
Residential Mortgage-Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	59,916
Unrealized Losses Less Than 12 Months	(1,678)
Fair Value 12 Months or Greater	150
Unrealized Losses 12 Months or Greater	(5)
Fair Value Total	60,066
Unrealized Losses Total	(1,683)
Commercial Mortgage Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	4,130
Unrealized Losses Less Than 12 Months	(221)
Fair Value 12 Months or Greater	6
Fair Value Total	4,136
Unrealized Losses Total	(221)
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	1,037
Unrealized Losses Less Than 12 Months	(18)
Fair Value Total	1,037
Unrealized Losses Total	$ (18)

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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