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Income Taxes - Unrecognized Tax Benefits (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Federal, State and Foreign Tax    
Beginning unrecognized tax balance $ 1.4 $ 1.7
Increase prior period positions 0.2 0.1
Increase current period positions 0.1 0.2
Decrease prior period positions   (0.3)
Decrease current period positions   (0.1)
Statutes expiring (0.2) (0.2)
Ending unrecognized tax benefits $ 1.5 $ 1.4