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Acquisitions - GCA Consideration Transferred (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
12 Months Ended
Sep. 01, 2017
Oct. 31, 2019
Oct. 31, 2018
Oct. 31, 2017
Business Acquisition, Equity Interests Issued or Issuable [Line Items]        
Fair value of ABM common stock at closing   $ 0.0 $ 0.0 $ 421.3
GCA Holding Corp.        
Business Acquisition, Equity Interests Issued or Issuable [Line Items]        
Fair value of ABM common stock at closing $ 421.3      
Cash consideration 837.5      
Total consideration transferred $ 1,258.8      
GCA Holding Corp. | Common Stock        
Business Acquisition, Equity Interests Issued or Issuable [Line Items]        
Shares of ABM common stock, net of shares withheld for taxes (in shares) 9.4      
ABM common stock closing market price at acquisition date (usd per share) $ 44.63