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Financial Instruments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 9 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]    
Corporate and municipal bonds $ 29.0 $ 28.2
Maturity dates of bonds, start date 2013-10  
Maturity dates of bonds, end date 2018-07