Income Taxes - U.S. Statutory Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Provision for income taxes at U.S. federal statutory income tax rate		$ 49.6	$ 50.1	$ 59.0
State and local income taxes, net of federal benefit		5.2	5.3	2.9
Net effects of foreign rate differential		2.6	1.8	(2.6)
Net effects of foreign operations		2.6	1.6	(7.4)
Net effect to deferred taxes for changes in tax rates		(1.1)	(2.6)	(0.3)
Net effect to deferred taxes for U.S. Tax Reform		0.0	0.8	(67.1)
Net effect of U.S. Tax Reform transition tax		0.0	(0.1)	0.2
Net effects of GILTI inclusion		6.4	6.5	0.0
Foreign derived intangible income deduction		(1.9)	(1.8)	0.0
Unrecognized tax benefits, net of federal benefit		(6.8)	(7.0)	1.1
Domestic production activities deduction		0.0	0.0	(3.2)
Research and development credit		(1.2)	(0.9)	(0.9)
Excess tax benefits related to equity compensation		(4.4)	(1.1)	(0.5)
§162(m) compensation limitation		2.7	1.9	1.0
Net changes in valuation allowance		0.0	(2.3)	(2.8)
Other	$ 0.6	0.4	1.2	1.1
Provision (benefit) for income taxes		$ 54.1	$ 53.4	$ (19.5)