Income Taxes - Deferred tax (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Deferred tax assets:
Compensation and benefit accruals not currently deductible	$ 119.4	$ 98.0
Net operating loss carryforwards	173.2	132.6
Self-insurance reserves	17.6	11.3
Research and experimentation and other tax credits	112.9	138.5
Pension liability	95.1	78.2
Accrued liabilities	303.2	97.2
Capital loss carryforward	104.8
Other	26.0	14.8
Total deferred tax assets	952.2	570.6
Deferred tax liabilities:
Unearned revenue	(40.3)	(53.4)
Depreciation and amortization	(106.7)	(76.3)
Acquired intangible assets	(24.5)	(25.1)
Investment in subsidiaries	(10.9)	(10.9)
Right of use assets	(164.9)
Contingent consideration	(33.6)
Total deferred tax liabilities	(380.9)	(165.7)
Valuation allowance	(217.5)	(120.6)
Net deferred tax assets	353.8	284.3
Income Taxes, additional disclosures
Remaining gross tax asset exclusive of tax liabilities realized	734.7
Change in valuation allowance	96.9
Valuation allowance related to capital losses.	71.2
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	65.8	70.1
Unrecognized tax benefits	47.1	55.7	$ 53.8
Unrecognized tax benefits that would be included in the effective tax rate if recognized	29.5
Capital loss carryforwards	355.7
Canada
Deferred tax liabilities:
Valuation allowance	(31.7)
Income Taxes, additional disclosures
Change in valuation allowance	31.7
Foreign net operating loss carryforwards	1.5
United States
Deferred tax liabilities:
Valuation allowance			$ (38.1)
California Franchise Tax Board \| Enterprise Zone Tax Credits
Income Taxes, additional disclosures
Unused tax credits	14.5
Federal
Income Taxes, additional disclosures
Liability for unrecognized tax benefits, including potential interest and penalties, net of related tax benefit	47.1	55.7
Federal \| Research and development
Income Taxes, additional disclosures
Unused tax credits	71.2
State
Income Taxes, additional disclosures
Net operating loss carryforwards	710.2
State \| Research and development
Income Taxes, additional disclosures
Unused tax credits	$ 27.2
Foreign
Income Taxes, additional disclosures
Net operating loss carryforwards		$ 505.3

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