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Income Taxes - Deferred tax (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016
Deferred tax assets:
Compensation and benefit accruals not currently deductible	$ 108.3		$ 108.3	$ 144.2
Net operating loss carryforwards	252.4		252.4	338.2
Self insurance reserves	13.5		13.5	23.8
Research and experimentation and other tax credits	178.1		178.1	167.5
Pension liability	88.2		88.2	142.1
Accrued liabilities	63.4		63.4	160.7
Other	27.8		27.8	25.0
Total deferred tax assets	731.7		731.7	1,001.5
Deferred tax liabilities:
Unearned revenue	(121.1)		(121.1)	(190.8)
Depreciation and amortization	(135.9)		(135.9)	(158.6)
Acquired intangible assets	(56.0)		(56.0)	(121.7)
Investment in subsidiaries	(109.5)		(109.5)	(241.2)
Total deferred tax liabilities	(422.5)		(422.5)	(712.3)
Valuation allowance	(197.1)		(197.1)	(138.4)
Net deferred tax assets	112.1		112.1	150.8
Income Taxes, additional disclosures
Remaining gross tax asset exclusive of tax liabilities realized	534.6		534.6
Change in valuation allowance			58.7
Change in valuation allowance for foreign tax credits			36.3
Foreign net operating loss carryforwards	7.7		7.7
Gross book-tax differences from non-U.S. subsidiaries	1,700.0		1,700.0
Liability for unrecognized tax benefits, including potential interest and penalties, net of related tax benefit	71.9		71.9	$ 109.5
Unrecognized tax benefits that would be included in the effective tax rate if recognized	42.4		42.4
United Kingdom
Income Taxes, additional disclosures
Change in valuation allowance		$ (59.9)
Canada
Deferred tax liabilities:
Valuation allowance	(13.1)		(13.1)
Income Taxes, additional disclosures
Change in valuation allowance	13.1
United States
Deferred tax liabilities:
Valuation allowance	(38.1)		(38.1)
California Franchise Tax Board | Enterprise Zone Tax Credits
Income Taxes, additional disclosures
Unused tax credits	6.8		6.8
Employee share-based payment adjustment
Income Taxes, additional disclosures
Net operating loss carryforwards					$ 3.8
Federal
Income Taxes, additional disclosures
Net operating loss carryforwards	50.3		50.3
Federal | Research and development
Income Taxes, additional disclosures
Unused tax credits	79.1		79.1
State
Income Taxes, additional disclosures
Net operating loss carryforwards	707.0		707.0
State | Research and development
Income Taxes, additional disclosures
Unused tax credits	33.4		33.4
Foreign
Income Taxes, additional disclosures
Net operating loss carryforwards	968.4		968.4
Unused tax credits	$ 61.8		$ 61.8

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