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Investments - Additional Information (Detail)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
USD ($)
Dec. 31, 2011
USD ($)
Dec. 31, 2010
USD ($)
Dec. 31, 2012
ABCP [Member]
USD ($)
Dec. 31, 2012
ABCP [Member]
CAD
Dec. 31, 2011
ABCP [Member]
CAD
Jan. 16, 2009
ABCP [Member]
CAD
Schedule of Equity Method Investments [Line Items]              
Investments in ABCP         107 125  
Restructuring notes in a Master Trust MAV2-A Notes issued under new restructuring plan             102
Restructuring notes in a MAV2-B and C Notes issued under new restructuring plan             9
Tracking notes deemed ineligible for inclusion in the Master Trust             23
Increase in carrying value of equity method investment       15      
Sales to equity method investees $ 171 $ 76 $ 18