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Fair Value Measurements - Additional Information (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2015
USD ($)
Source
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of independent pricing sources used to value privately issued mortgage-backed securities valued with internal modeling | Source 2    
Securities issued by financial institutions and other entities backed by trust preferred securities at cost $ 28 $ 30  
Fair value of securities issued by financial institutions and other entities backed by trust preferred securities 47 50  
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 75 55 $ 58
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 210 173 222
Assets taken in foreclosure of defaulted loans measured at fair value on a nonrecurring basis 29 19  
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 106 94 173
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 $ 104 $ 79 $ 49
Minimum [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Discount margin over LIBOR 4.00%    
Discount rates for fair value estimations 10.00%    
Maximum [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Discount margin over LIBOR 10.00%    
Discount rates for fair value estimations 90.00%    
Weighted Average [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Discount margin over LIBOR 8.00%