Revenues - Consolidated Statements of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2019	Oct. 31, 2018	Oct. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 1,648.0	$ 1,647.9	$ 1,594.7	$ 1,607.9	$ 1,648.8	$ 1,624.3	$ 1,580.8	$ 1,588.3	$ 6,498.6	$ 6,442.2	$ 5,453.6
Operating expenses									5,767.5	5,747.4	4,881.2
Selling, general and administrative expenses									452.9	438.0	436.6
Income tax provision									32.7	(8.2)	8.8
Net income	$ 47.9	$ 36.8	$ 29.7	$ 13.0	$ 9.7	$ 33.6	$ 26.6	$ 27.8	$ 127.4	$ 97.8	$ 3.8
Net income per common share — Basic (in usd per share)	$ 0.72	$ 0.55	$ 0.45	$ 0.20	$ 0.15	$ 0.51	$ 0.40	$ 0.42	$ 1.91	$ 1.48	$ 0.07
Net income per common share — Diluted (in usd per share)	$ 0.71	$ 0.55	$ 0.45	$ 0.19	$ 0.15	$ 0.51	$ 0.40	$ 0.42	$ 1.90	$ 1.47	$ 0.07
Under Historical Guidance
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									$ 6,546.2
Operating expenses									5,816.2
Selling, general and administrative expenses									459.7
Income tax provision									30.7
Net income									$ 121.6
Net income per common share — Basic (in usd per share)									$ 1.83
Net income per common share — Diluted (in usd per share)									$ 1.82
Effect of Adoption | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									$ (47.6)
Operating expenses									(48.6)
Selling, general and administrative expenses									(6.7)
Income tax provision									2.0
Net income									$ 5.8
Net income per common share — Basic (in usd per share)									$ 0.09
Net income per common share — Diluted (in usd per share)									$ 0.09