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Fair Value Measurement- Quantitative Information about Level 3 Fair Value Measurements of Assets (Detail) - Fair Value, Measurements, Recurring - Level 3 - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2015
Dec. 31, 2014
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair value of assets $ 40 $ 55
Securities available-for-sale | State and political subdivisions | Discounted Cash Flow    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair value of assets $ 11  
Fair value inputs, expected credit loss (percent) 1.00%  
Trading Securities | Structured Foreign Exchange Swaptions | Option Pricing Model | Minimum    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Correlation risk (percent) [1] 0.00%  
Volatility (percent) 11.00%  
Trading Securities | Structured Foreign Exchange Swaptions | Option Pricing Model | Maximum    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Correlation risk (percent) [1] 25.00%  
Volatility (percent) 13.00%  
Trading Securities | Long-Term Foreign Exchange Options | Option Pricing Model    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair value of assets [1] $ 1  
[1] The option pricing model uses market inputs such as foreign currency exchange rates, interest rates and volatility to calculate the fair value of the option.