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Stock-Based Compensation - Summary of Characteristics of Options Outstanding (Detail)
In Millions, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
USD ($)
Dec. 31, 2012
CAD
Dec. 31, 2012
Range One [Member]
USD ($)
Dec. 31, 2012
Range Two [Member]
USD ($)
Dec. 31, 2012
Range Three [Member]
USD ($)
Dec. 31, 2012
Range Four [Member]
USD ($)
Dec. 31, 2012
Range Five [Member]
USD ($)
Dec. 31, 2012
Range Six [Member]
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Exercise price range lower limit     $ 15 $ 25 $ 35 $ 40 $ 45 $ 50
Exercise price range upper limit     $ 20 $ 30 $ 40 $ 45 $ 50 $ 55
Number of options 6,623,242 6,623,242 986,868 3,050,032 60,000 27,342 1,384,000 1,115,000
Remaining contractual life [years] 4 years 6 months 15 days 4 years 6 months 15 days 3 years 2 months 12 days 4 years 2 months 12 days 4 years 6 months 2 years 9 months 18 days 5 years
Number of options exercisable 3,227,574 3,227,574 986,868 1,400,032 60,000 27,342 10,000 743,332
Aggregate intrinsic value $ 93              
Weighted average exercise price of options   35.39            
Weighted average life remaining [years] 4 years 4 years            
Weighted average exercise price   30.93            
Aggregate intrinsic value of options exercisable $ 60