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Employee Benefit Plans - Schedule of Combined Funded Status and Net Periodic Pension Cost and Postretirement Benefit - Pension 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 Fair Value of Plan Assets During the Year:          
Employer contributions     $ 21.1    
Amounts recognized in Consolidated Balance Sheets          
Noncurrent liability $ 124.6   152.4 $ 182.1  
Defined Benefit Pension Plan [Member]          
Change in Projected Benefit Obligation During the Year:          
Beginning balance 593.0 $ 605.9 605.9 636.0  
Service cost     0.0 0.0  
Interest cost 12.3 12.9 17.2 18.2 $ 23.3
Actuarial loss     20.3 4.3  
Benefits paid and plan expenses     (50.4) (52.6)  
Ending balance     593.0 605.9 636.0
Change in Fair Value of Plan Assets During the Year:          
Beginning balance $ 438.6 $ 416.4 416.4 410.1  
Actual return on plan assets     49.6 20.5  
Employer contributions     23.0 38.4  
Benefits paid and plan expenses     (50.4) (52.6)  
Ending balance     438.6 416.4 $ 410.1
Funded status of plans     (154.4) (189.5)  
Amounts recognized in Consolidated Balance Sheets          
Current liability     20.3 27.1  
Noncurrent liability     134.1 162.4  
Amounts recognized in Accumulated Other Comprehensive Loss          
Accumulated other comprehensive loss, net of tax     $ 151.4 $ 167.2