Quarterly report pursuant to Section 13 or 15(d)

Income Tax Benefit/Expense - Additional Information (Detail)

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Income Tax Benefit/Expense - Additional Information (Detail) (USD $)
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Income Tax Expense Benefit [Line Items]        
Income tax expense $ 6,695,000 $ 15,042,000 $ 31,155,000 $ 33,345,000
Reduction in pre-tax earnings $ 17,500,000 $ 35,700,000