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Derivative Financial Instruments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2012
Jul. 25, 2011
Sep. 30, 2012
Interest rate swaps [Member]
Contract
Dec. 31, 2011
Interest rate swaps [Member]
Contract
Sep. 30, 2012
Natural gas swaps [Member]
Contract
Dec. 31, 2011
Natural gas swaps [Member]
Contract
Sep. 30, 2012
$1.0 Billion credit facility [Member]
Derivative Instruments, Gain (Loss) [Line Items]              
Derivative contracts held     0 0 0 0  
Interest rate swaps associated with term loan borrowings   $ 500.0          
Percentage of variable rate debt outstanding   71.00%          
Company's credit facility $ 925 $ 1,000          
Weighted average fixed rate             3.94% plus the applicable margin on these borrowings
Base rate added to applicable margin of borrowings             3.94%