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Employee Benefit Plans - Schedule of Combined Funded Status and Net Periodic Pension Cost and Postretirement Benefit - Postretirement Benefits (Detail) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Change in Plan Assets:          
Company contributions     $ 21.1    
Amounts recognized in Consolidated Balance Sheets          
Noncurrent liability $ 124.6   152.4 $ 182.1  
Other Postretirement Benefits Plan [Member]          
Change in Benefit Obligation:          
Beginning balance 19.6 $ 21.0 21.0 23.2  
Interest cost 0.3 0.3 0.5 0.6 $ 0.9
Participant contributions     1.1 1.2  
Actuarial gain     (0.7) (1.4)  
Benefits paid     (2.3) (2.6)  
Ending balance     19.6 21.0 23.2
Change in Plan Assets:          
Beginning balance $ 0.0 $ 0.0 0.0 0.0  
Company contributions     1.2 1.5  
Participant contributions     1.1 1.1  
Benefits paid     (2.3) (2.6)  
Ending balance     0.0 0.0 $ 0.0
Funded Status     (19.6) (21.0)  
Amounts recognized in Consolidated Balance Sheets          
Current liability     2.4 2.6  
Noncurrent liability     17.2 18.4  
Amounts recognized in Accumulated Other Comprehensive Loss          
Accumulated other comprehensive income, net of tax     $ (8.9) $ (10.9)