Income Taxes - Deferred tax (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Deferred tax assets:
Compensation and benefit accruals not currently deductible	$ 130.0	$ 119.4
Net operating loss carryforwards	169.8	173.2
Self-insurance reserves	12.3	17.6
Research and experimentation and other tax credits	117.2	112.9
Pension liability	87.7	95.1
Accrued liabilities	284.7	307.6
Capital loss carryforward	62.0	104.8
Other	37.3	26.0
Total deferred tax assets	901.0	956.6
Deferred tax liabilities:
Unearned revenue	(19.8)	(40.3)
Depreciation and amortization	(116.5)	(106.7)
Acquired intangible assets	(19.4)	(24.5)
Investment in subsidiaries	(13.4)	(10.9)
Right of use assets	(145.6)	(164.9)
Contingent consideration	(33.8)	(33.6)
Total deferred tax liabilities	(348.5)	(380.9)
Valuation allowance	197.7	217.5
Net deferred tax assets	354.8	358.2
Income Taxes, additional disclosures
Remaining gross tax asset exclusive of tax liabilities realized	703.3
Change in valuation allowance	19.8
Valuation allowance related to capital losses.	49.5
Change in valuation allowance for foreign tax credits	29.6
Gross book-tax differences from non-U.S. subsidiaries	1,500.0
Liability for unrecognized tax benefits, including potential interest and penalties, net of related tax benefit	62.8	65.8
Unrecognized tax benefits	46.4	47.1	$ 55.7
Unrecognized tax benefits that would be included in the effective tax rate if recognized	40.1
Capital loss carryforwards	184.1	355.7
Canada
Deferred tax liabilities:
Valuation allowance		31.7
United States
Deferred tax liabilities:
Valuation allowance			$ 38.1
California Franchise Tax Board \| Enterprise Zone Tax Credits
Income Taxes, additional disclosures
Unused tax credits	18.1
Federal
Income Taxes, additional disclosures
Liability for unrecognized tax benefits, including potential interest and penalties, net of related tax benefit	46.4	47.1
Federal \| Research and development
Income Taxes, additional disclosures
Unused tax credits	75.7
State
Income Taxes, additional disclosures
Net operating loss carryforwards	667.0
State \| Research and development
Income Taxes, additional disclosures
Unused tax credits	$ 23.4
Foreign
Income Taxes, additional disclosures
Net operating loss carryforwards		$ 710.2

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