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Fair Value Measurements - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
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 $ 27 $ 42  
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 178 210 $ 210
Assets taken in foreclosure of defaulted loans measured at fair value on a nonrecurring basis 19 32  
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 97 88 145
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 $ 81 $ 122 $ 65