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Share-Based Compensation - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Apr. 24, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 20, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based compensation expense   $ 19.5 $ 12.8        
Performance Shares [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based compensation expense   4.8          
Share-based compensation expense related to unvested performance-based stock options not yet recognized   0.0   $ 5.3      
Term Based Stock Options [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based compensation expense related to unvested performance-based stock options not yet recognized   $ 49.7   $ 21.7      
Share-based compensation expense related to unvested performance-based stock options expected to be recognized over a weighted average period   1 year 10 months 24 days   1 year 3 months 18 days      
Stock options, vested       6,819,649      
Restricted Stock Units (RSUs) [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based compensation expense related to unvested performance-based stock options expected to be recognized over a weighted average period   2 years 9 months 18 days   2 years 2 months 12 days      
Stock options, granted   159,323   76,192      
Stock options, vested   230,990          
Shares issued upon vesting of RSUs   105,990          
Remaining vested and releasable   125,000          
Unvested restricted stock units outstanding   534,323   605,990      
Share-based compensation expense related to unvested performance-based stock options not yet recognized   $ 10.6   $ 7.1      
Restricted Stock Units (RSUs) [Member] | IPO [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Stock options, vested   29,800          
Restricted Stock Units (RSUs) [Member] | Minimum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Restricted stock vesting period   3 years   3 years      
Restricted Stock Units (RSUs) [Member] | Maximum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Restricted stock vesting period   4 years   4 years      
Hcm Stock Incentive Plan 2007 [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Number of shares authorized   10,540,540   10,540,540      
Common shares outstanding   5,000   10,000      
Hcm Stock Incentive Plan 2013 [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Number of shares authorized   12,500,000   12,500,000     15,000,000
Common shares outstanding   10,766,257          
Share-based compensation expense       $ 16.1 $ 12.5 $ 12.8  
Share based compensation arrangement by share based payment award exercise period after employment termination   90 days   90 days      
Stock option awards, contractual term   10 years   10 years      
Shares available for future grants of stock options and stock awards       1,939,403      
Hcm Stock Incentive Plan 2013 [Member] | Minimum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Restricted stock vesting period   4 years   4 years      
Hcm Stock Incentive Plan 2013 [Member] | Maximum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Restricted stock vesting period   5 years   5 years      
Hcm Stock Incentive Plan 2013 [Member] | Stock Appreciation Rights (SARs) [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Number of shares authorized   600,000   600,000      
Common shares outstanding   0   266,350      
Share-based compensation expense   $ 1.5          
Hcm Stock Incentive Plan 2013 [Member] | Performance Shares [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based compensation vesting threshold       The performance-based options vest on the earlier to occur of a change in control or an initial public offering ("IPO") in which the value of Ceridian stock is at least $26.92 per share or higher. If the value of the common stock has not reached $26.92 or higher per share at the time an IPO or change in control event occurs, the options expire unvested, or through the expiration date. Options issued under the 2013 HCM SIP, other than the Replacement Options, do not include performance based-options.      
Hcm Stock Incentive Plan 2013 [Member] | Term Based Stock Options [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term       7 years      
Hcm Equity Incentive Plan 2018 [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Number of shares authorized 13,500,000            
Common shares outstanding   5,125,110          
Restricted stock vesting period 4 years            
Share based compensation arrangement by share based payment award exercise period after employment termination 90 days            
Stock option awards, contractual term 10 years            
Shares available for future grants of stock options and stock awards   8,374,890