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Income Taxes - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2013
Mar. 31, 2012
Sep. 30, 2012
Income Taxes [Line Items]          
Effective income tax rate 27.10% 32.60% 30.60% 31.80%  
Unrecognized tax benefits, uncertain tax positions $ 8.4   $ 8.4   $ 7.5
Unrecognized tax benefits, accrued interest $ 0.7   $ 0.7   $ 0.7