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Fair Value of Financial Instruments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2014
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Dealer marks of securities 81.00% 81.00%
Percentage of carrying value for which dealer quotes have been received 94.00% 94.00%
Percentage difference of internal valuation than dealer marks 2.00% 2.00%
Commercial Loans
    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair value option elected aggregate carrying amount, asset $ 340 $ 609
Commercial Secured Borrowings
    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair value option elected aggregate carrying amount, asset   65
Asset-backed Securities
    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Dealer marks of securities 42.00% 42.00%
Percentage difference of internal valuation than dealer marks 1.00% 1.00%
Senior Securities | Residential
    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Fair value option elected aggregate carrying amount, asset $ 3,310 $ 6,120
Mortgage Servicing Rights | Maximum
    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Percentage difference of internal valuation than dealer marks 1.00% 1.00%