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Fair Value Measurements - Fair Value of Investments in Marketable Securities (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Fair Value Of Other Financial Instrument [Line Items]    
Total investments $ 157.8 $ 155.2
Equity securities [Member]
   
Fair Value Of Other Financial Instrument [Line Items]    
Total investments 3.2 2.8
Government bonds [Member]
   
Fair Value Of Other Financial Instrument [Line Items]    
Total investments 111.9 111.4
Other liquid investments [Member]
   
Fair Value Of Other Financial Instrument [Line Items]    
Total investments $ 42.7 $ 41.0