Acquisitions - GCA Consideration Transferred (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions			12 Months Ended
		Sep. 01, 2017	Oct. 31, 2018	Oct. 31, 2017	Oct. 31, 2016
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Fair value of ABM common stock at closing			$ 0.0	$ 421.3	$ 0.0
GCA Holding Corp.
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Fair value of ABM common stock at closing		$ 421.3
Cash consideration	[1]	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

[1] Revised during the second quarter of 2018 to reflect a post-closing purchase price adjustment related to a net working capital settlement.