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Financial Data by Segment and Geographic Area - Schedule of Revenue by Country (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
Segment Reporting, Revenue Reconciling Item [Line Items]          
Total revenue $ 546.1 $ 488.4 $ 670.8 $ 623.4 $ 611.6
United States [Member]          
Segment Reporting, Revenue Reconciling Item [Line Items]          
Total revenue     465.2 432.7 420.0
Canada [Member]          
Segment Reporting, Revenue Reconciling Item [Line Items]          
Total revenue     203.4 189.0 190.8
Other Country [Member]          
Segment Reporting, Revenue Reconciling Item [Line Items]          
Total revenue     $ 2.2 $ 1.7 $ 0.8