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Summary of Significant Accounting Policies - Impact of Adoption of ASC 606 on Revenue (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
Recurring services $ 480.8 $ 438.2 $ 598.5 $ 558.5 $ 559.3
Professional services and other 65.3 50.2 72.3 64.9 52.3
Total revenue 546.1 488.4 670.8 623.4 611.6
Operating profit (loss) 31.3 16.9 $ 33.0 $ (11.2) $ (16.3)
Accounting Standards Update 2014-09 [Member]          
Recurring services 460.4 419.8      
Professional services and other 85.5 72.7      
Total revenue 545.9 492.5      
Operating profit (loss) 36.9 25.7      
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]          
Recurring services (20.4) (18.4)      
Professional services and other 20.2 22.5      
Total revenue (0.2) 4.1      
Operating profit (loss) $ 5.6 $ 8.8