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Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail)
$ in Millions
Sep. 30, 2020
USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value Less Than 12 Months $ 8,744
Unrealized Losses Less Than 12 Months (32)
Fair Value 12 Months or Greater 1,201
Unrealized Losses 12 Months or Greater (2)
Fair Value Total 9,945
Unrealized Losses Total (34)
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 1,322
Unrealized Losses Less Than 12 Months (6)
Fair Value Total 1,322
Unrealized Losses Total (6)
Residential Mortgage-Backed Securities [Member]  
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value Less Than 12 Months 5,995
Unrealized Losses Less Than 12 Months (20)
Fair Value 12 Months or Greater 1,195
Unrealized Losses 12 Months or Greater (2)
Fair Value Total 7,190
Unrealized Losses Total (22)
Commercial Mortgage Backed Securities [Member]  
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value Less Than 12 Months 1,170
Unrealized Losses Less Than 12 Months (4)
Fair Value Total 1,170
Unrealized Losses Total (4)
Obligations of State and Political Subdivisions [Member]  
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value Less Than 12 Months 255
Unrealized Losses Less Than 12 Months (2)
Fair Value Total 255
Unrealized Losses Total (2)
Obligations of Foreign Governments [Member]  
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value Less Than 12 Months 2
Fair Value Total 2
Corporate Debt Securities [Member]  
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value 12 Months or Greater 4
Fair Value Total $ 4