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Fair Value Measurements - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Fair Value Disclosures [Abstract]    
Minimum days to maturity of investments 90 days  
Long-term debt, fair value $ 20,652.8  
Carrying value of outstanding debt $ 21,412.1 $ 19,799.9