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Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail)
$ in Millions
Mar. 31, 2019
USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value Less Than 12 Months $ 1,635
Unrealized Losses Less Than 12 Months (6)
Fair Value 12 Months or Greater 31,906
Unrealized Losses 12 Months or Greater (655)
Fair Value Total 33,541
Unrealized Losses Total (661)
Fair Value Less Than 12 Months 4,491
Unrealized Losses Less Than 12 Months (18)
Fair Value 12 Months or Greater 43,263
Unrealized Losses 12 Months or Greater (782)
Fair Value Total 47,754
Unrealized Losses Total (800)
U.S. Treasury and Agencies [Member]  
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value 12 Months or Greater 4,449
Unrealized Losses 12 Months or Greater (92)
Fair Value Total 4,449
Unrealized Losses Total (92)
Fair Value Less Than 12 Months 21
Fair Value 12 Months or Greater 17,478
Unrealized Losses 12 Months or Greater (225)
Fair Value Total 17,499
Unrealized Losses Total (225)
Residential Mortgage-Backed Securities [Member] | Agency [Member]  
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value Less Than 12 Months 1,632
Unrealized Losses Less Than 12 Months (6)
Fair Value 12 Months or Greater 27,456
Unrealized Losses 12 Months or Greater (563)
Fair Value Total 29,088
Unrealized Losses Total (569)
Fair Value Less Than 12 Months 4,470
Unrealized Losses Less Than 12 Months (18)
Fair Value 12 Months or Greater 24,097
Unrealized Losses 12 Months or Greater (523)
Fair Value Total 28,567
Unrealized Losses Total (541)
Commercial [Member] | Agency [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 12 Months or Greater 1,686
Unrealized Losses 12 Months or Greater (34)
Fair Value Total 1,686
Unrealized Losses Total (34)
Asset-Backed Securities [Member] | Other Asset-Backed Securities [Member]  
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value 12 Months or Greater 1
Fair Value Total 1
Obligations of Foreign Governments [Member]  
Continuous Gross Unrealized Losses And Fair Value [Abstract]  
Fair Value Less Than 12 Months 3
Fair Value Total $ 3