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Fair Value Measurements - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]          
Measurement Input [Extensible List] us-gaap:MeasurementInputComparabilityAdjustmentMember   us-gaap:MeasurementInputComparabilityAdjustmentMember    
Minimum [Member]          
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]          
Discount rates for fair value estimations     15.00%    
Maximum [Member]          
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]          
Discount rates for fair value estimations     90.00%    
Significant Unobservable Inputs (Level 3) [Member]          
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]          
Change in fair value of nonrecurring fair value measured loans for charge-offs and impairment reserves $ 35 $ 82 $ 125 $ 153  
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Loans [Member]          
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]          
Capitalized servicing rights 147   147   $ 159
Capitalized servicing rights, valuation allowance     29   $ 30
Valuation Charge Recognized 0 0 $ (1) 20  
Weighted-average prepayment speeds     14.96%   16.01%
Discounted rate represented weighted-average option-adjusted basis spread point percentage     900.00%   900.00%
Fair Value, Measurements, Nonrecurring [Member]          
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]          
Loans measured at fair value on nonrecurring basis 591 387 $ 591 387 $ 652
Assets taken in foreclosure of defaulted loans measured at fair value on a nonrecurring basis 4 27 4 27  
Fair Value, Measurements, Nonrecurring [Member] | Significant Observable Inputs (Level 2) [Member]          
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]          
Loans measured at fair value on nonrecurring basis 330 159 330 159 339
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]          
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]          
Loans measured at fair value on nonrecurring basis $ 261 $ 228 $ 261 $ 228 $ 313