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Investment Securities - Schedule of Available-for-Sale Securities (Parenthetical) (Detail) (USD $)
3 Months Ended 12 Months Ended
Jun. 30, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Fair Value $ 62,602,000,000 $ 63,979,000,000
Percentage of the securities in our asset-backed security portfolio were rated AAA or its equivalent 87.00% 82.00%
Fannie Mae, Freddie Mac and Ginnie Mae [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Fair Value 100,000,000 302,000,000
Agency debt obligations, amortized cost of debt securities 100,000,000 300,000,000
Minimum Individual Percentage of Share in stockholders Equity percent instant 10.00% 10.00%
Fannie Mae [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Mortgage-backed securities, amortized costs of debt securities 26,000,000,000 22,900,000,000
Mortgage-backed securities, fair values of debt securities 25,200,000,000 23,200,000,000
Freddie Mac [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Mortgage-backed securities, amortized costs of debt securities 12,500,000,000 12,600,000,000
Mortgage-backed securities, fair values of debt securities 12,300,000,000 12,900,000,000
Ginnie Mae [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Mortgage-backed securities, amortized costs of debt securities 8,000,000,000 9,900,000,000
Mortgage-backed securities, fair values of debt securities $ 7,900,000,000 $ 10,000,000,000
Credit Card and Installment Loans [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Percentage distribution of securities collateralized 66.00% 64.00%
Auto Dealer Floor Plan Inventory Loans and Leases [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Percentage distribution of securities collateralized 19.00% 18.00%
Auto Loan [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Percentage distribution of securities collateralized 7.00% 6.00%
Equipment Loans [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Percentage distribution of securities collateralized 6.00% 5.00%
Student Loans [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Percentage distribution of securities collateralized 1.00% 1.00%
Commercial Paper [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Percentage distribution of securities collateralized   2.00%
Other Loan [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Percentage distribution of securities collateralized 1.00% 4.00%