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Employee Benefit Plans - Components of Net Periodic Cost for Defined Benefit Pension Plan and Other Postretirement Benefit Plan (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
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Net periodic pension cost (postretirement benefit gain) $ 1.8 $ 0.9 $ 1.5 $ 3.0 $ 8.9
Defined Benefit Pension Plan [Member]          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Service cost     0.0 0.0  
Interest cost 12.3 12.9 17.2 18.2 23.3
Actuarial loss (gain) amortization 10.8 9.6 12.8 12.5 13.3
Expected return on plan assets (19.5) (19.8) (26.3) (25.7) (26.6)
Net periodic pension cost (postretirement benefit gain) 3.6 2.7 3.7 5.0 10.0
Other Postretirement Benefits Plan [Member]          
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]          
Service cost     0.0 0.0 0.0
Service benefit (0.3) (0.3)      
Interest cost 0.3 0.3 0.5 0.6 0.9
Actuarial loss (gain) amortization (1.8) (1.8) (2.4) (2.3) (1.7)
Prior service credit amortization     (0.3) (0.3) (0.3)
Net periodic pension cost (postretirement benefit gain) $ (1.8) $ (1.8) $ (2.2) $ (2.0) $ (1.1)