Income Taxes - Deferred tax (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Deferred tax assets:
Compensation and benefit accruals not currently deductible	$ 85.4	$ 92.2
Net operating loss carryforwards	107.5	102.3
Self-insurance reserves	38.3	23.2
Research and experimentation and other tax credits	67.1	43.1
Pension liability	34.9	44.7
Accrued liabilities	265.7	295.1
Capital loss carryforward	49.5	64.0
Partnership investment	22.3	102.0
Other	8.2	7.1
Total deferred tax assets	678.9	773.7
Deferred tax liabilities:
Unearned revenue	(4.1)	(7.0)
Depreciation and amortization	(83.7)	(13.1)
Acquired intangible assets		(5.4)
Investment in subsidiaries	(9.9)	(10.7)
Right of use assets	(85.2)	(94.0)
Contingent consideration	(30.5)	(34.2)
Other	(5.4)	(15.5)
Total deferred tax liabilities	(218.8)	(179.9)
Valuation allowance	(160.9)	(171.2)
Net deferred tax assets	299.2	422.6
Income Taxes, additional disclosures
Net operating loss carryforwards	744.6	757.5
Gross tax asset exclusive of tax liabilities realizable	518.0
Increase (decrease) in valuation allowance	(10.3)
Gross book-tax differences from non-U.S. subsidiaries	1,200.0
Liability for unrecognized tax benefits, including potential interest and penalties, net of related tax benefit	97.9	79.5
Unrecognized tax benefits	81.3	62.1	$ 55.2
Unrecognized tax benefits that would be included in the effective tax rate if recognized	77.1
ACAP investment sale
Deferred tax liabilities:
Valuation allowance	(9.3)
Income Taxes, additional disclosures
Increase (decrease) in valuation allowance	(21.0)
Capital losses
Income Taxes, additional disclosures
Increase (decrease) in valuation allowance	(10.4)
Foreign net operating losses and currency translation adjustments
Income Taxes, additional disclosures
Increase (decrease) in valuation allowance	12.3
State net operating losses and credits
Income Taxes, additional disclosures
Increase (decrease) in valuation allowance	(0.6)
UNITED STATES \| ACAP investment sale
Income Taxes, additional disclosures
Increase (decrease) in valuation allowance	(11.7)
Capital loss carryforwards
Income Taxes, additional disclosures
Tax credit carryforward	181.2	$ 199.4
Other credits
Income Taxes, additional disclosures
Tax credit carryforward	9.7
Federal \| Research and development credits
Income Taxes, additional disclosures
Tax credit carryforward	28.4
State \| Research and development credits
Income Taxes, additional disclosures
Tax credit carryforward	27.8
Foreign \| Research and development credits
Income Taxes, additional disclosures
Tax credit carryforward	$ 4.6

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from investments (excludes investments in subsidiaries and equity method investments).

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from pension benefits.

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Amount of deferred tax liability attributable to taxable temporary differences from intangible assets other than goodwill.

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Amount of deferred tax liability attributable to taxable temporary differences from property, plant, and equipment.

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Amount of deferred tax liability attributable to taxable temporary differences from undistributed earnings of subsidiary and other recognized entity not within country of domicile. Includes, but is not limited to, other basis differences.

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