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Quarterly Financial Data - Additional Information (Detail) - Tradenames [Member] - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Sep. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Quarterly Financial Information [Line Items]            
Pre-tax defined benefit plan actuarial gains (losses) $ (0.8) $ 1.3 $ (1.0) $ (0.9) $ 0.5 $ (1.9)
After-tax defined benefit plan actuarial gains (losses) $ (0.5) $ 0.9 $ (0.7) $ (0.6)