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Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Income Taxes    
Balance at January 1 $ 4.2 $ 2.6
Additions for tax positions of prior years 2.0 1.6
Balance at December 31 $ 6.2 $ 4.2