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Employee Benefit Plans - Schedule of Combined Funded Status and Net Periodic Pension Cost and Postretirement Benefit - Postretirement Benefits (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Other Postretirement Benefits Plan [Member]    
Accumulated other comprehensive loss, tax $ (9.9) $ (9.9)