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Net Loss per Share - Schedule of Numerators and Denominators of Basic and Diluted Net Loss per Share Computations (Detail) - USD ($)
$ / shares in Units, $ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Numerator:          
Net income (loss) attributable to Ceridian $ (63.2) $ (54.2) $ (9.2) $ (92.9) $ (104.7)
Less: Income (loss) from discontinued operations (14.8) (2.4) (6.0) 12.5 3.7
Net income (loss) from continuing operations attributable to Ceridian (48.4) (51.8) (3.2) (105.4) (108.4)
Less: Senior Preferred Stock dividends declared 7.7 15.3 20.5 14.1  
Net income (loss) from continuing operations attributable to Ceridian available to common stockholders $ (56.1) $ (67.1) $ (23.7) $ (119.5) $ (108.4)
Denominator:          
Weighted-average shares outstanding - basic and diluted 105,730,178 65,181,373 65,204,960 64,988,338 64,924,845
Weighted-average shares outstanding-basic 105,730,178 65,181,373 65,204,960 64,988,338 64,924,845
Weighted-average shares outstanding-diluted 105,730,178 65,181,373 65,204,960 64,988,338 64,924,845
Net income (loss) per share from continuing operations attributable to Ceridian-basic $ (0.53) $ (1.03)      
Net loss per share from continuing operations attributable to Ceridian-basic and diluted     $ (0.36) $ (1.84) $ (1.67)
Net loss per share from discontinued operations-basic (0.14) (0.04)      
Net (loss) income per share from discontinued operations-basic and diluted     (0.10) 0.19 0.06
Net income (loss) per share attributable to Ceridian-basic (0.67) (1.07)      
Net loss per share attributable to Ceridian-basic and diluted     $ (0.46) $ (1.65) $ (1.61)
Net income (loss) per share from continuing operations attributable to Ceridian-diluted (0.53) (1.03)      
Net loss per share from discontinued operations-diluted (0.14) (0.04)      
Net income (loss) per share attributable to Ceridian- diluted $ (0.67) $ (1.07)