Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Mar. 31, 2020 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 4,953
Unrealized Losses Less Than 12 Months	(70)
Fair Value 12 Months or Greater	6,152
Unrealized Losses 12 Months or Greater	(82)
Fair Value Total	11,105
Unrealized Losses Total	(152)
Other Asset-Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	2
Fair Value Total	2
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	47
Unrealized Losses Less Than 12 Months	(1)
Fair Value Total	47
Unrealized Losses Total	(1)
Residential Mortgage-Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	4,716
Unrealized Losses Less Than 12 Months	(68)
Fair Value 12 Months or Greater	6,150
Unrealized Losses 12 Months or Greater	(82)
Fair Value Total	10,866
Unrealized Losses Total	(150)
Obligations of State and Political Subdivisions [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	186
Unrealized Losses Less Than 12 Months	(1)
Fair Value Total	186
Unrealized Losses Total	(1)
Corporate Debt Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	4
Fair Value Total	$ 4

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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