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Retirement Benefits - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
9 Months Ended 12 Months Ended 12 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Pension Plans
Sep. 30, 2013
Pension Plans
Dec. 31, 2012
Other Postretirement Benefit Plans
Sep. 30, 2013
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]          
Expected contribution for next fiscal year   $ 7.0   $ 0.9  
Actual contributions     6.0   0.6
Anticipated additional contribution for pension plans and other postretirement benefit plans in 2013 $ 0.5