Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			19.00%	19.30%	20.00%
Gross unrecognized tax benefits		$ 13.8	$ 51.4	$ 13.8	$ 46.3	$ 25.0
Amount of tax benefits that, if recognized, would impact the effective tax rate			50.3
Payment of penalties		0.3	0.5	0.3
Payment of interest expense		2.9	3.6	$ 2.9
Foreign credit carryforwards			29.6
Tax loss carryforwards			2,911.9
Deferred tax assets, valuation allowance			692.3
Deferred Tax Assets, Operating Loss Carryforwards, Foreign			2,818.2
Deferred Tax Assets, Operating Loss Carryforwards, Foreign (jurisdictions with unlimited tax loss carryforward periods)			2,345.7
Deferred Tax Assets, Operating Loss Carryforwards, Domestic			93.7
State tax losses available for carry forward			56.1
Tax Cuts and Jobs Act of 2017, Corporate Federal Tax Rate				21.00%
Provisional income tax expense		$ 2.2
Measurement period adjustment	$ 3.6
Income tax decrease			1.4
Decrease of income tax expense related to remeasurement of deferred tax assets and liabilities			28.0
Transition tax expense			26.6
Minimum
Income Taxes [Line Items]
Possible amount of decrease during the next twelve months primarily as a result of the resolution of federal, state and foreign examinations and the expiration of various statutes of limitations			0.0
Maximum
Income Taxes [Line Items]
Possible amount of decrease during the next twelve months primarily as a result of the resolution of federal, state and foreign examinations and the expiration of various statutes of limitations			$ 8.1