Income Taxes - Narrative (Details) - USD ($)	3 Months Ended			6 Months Ended		12 Months Ended
	Apr. 30, 2018	Jan. 31, 2018	Apr. 30, 2017	Apr. 30, 2018	Apr. 30, 2017	Oct. 31, 2018
Income Taxes [Line Items]
Net discrete tax benefit from remeasurement of deferred taxes at new rate due to Tax Act		$ 28,700,000
One-time transition tax on deemed repatriation of indefinitely reinvested earnings		$ 7,000,000
Tax deductions for energy efficient government buildings	$ 2,000,000			$ 2,300,000
Increase (decrease) in tax credits, work opportunity tax credit	$ 1,500,000			1,500,000
Discrete tax benefit related to enactment of Tax Act				21,700,000
Income tax benefit related to vesting of share-based compensation awards			$ 2,100,000	$ 1,500,000	$ 2,100,000
Forecast
Income Taxes [Line Items]
U.S. statutory federal rate (percent)						23.30%