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Asset Impairment Charges - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2013
Jun. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Asset Impairment Charges [Line Items]        
Asset impairment charge $ 21.2 $ 6.2 $ 0 $ 21.2
Market recovery projection period     3 years