Income Taxes - Income Tax (Benefit) Provision from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Pretax (loss) income:
U.S.		$ (485.2)	$ (593.4)	$ (127.3)
Foreign		635.6	797.9	864.6
Income before income taxes		150.4	204.5	737.3
Current (benefit) expense:
U.S. Federal		21.1	(98.0)	369.4
State		1.0	(5.3)	0.5
Foreign		48.0	14.1	60.8
Total current (benefit) expense		70.1	(89.2)	430.7
Deferred expense (benefit):
U.S. Federal		(127.8)	11.9	82.5
State		(13.2)	0.6	0.1
Foreign		(349.3)	(74.7)	(31.4)
Total deferred (benefit) expense		(490.3)	(62.2)	51.2
Total Income tax (benefit) provision		(420.2)	(151.4)	481.9
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Computed expected income tax provision		31.5	43.0	232.6
State income taxes, net of federal benefit		(5.4)	(8.7)	(1.3)
Foreign income taxed at lower than the federal rate		(78.8)	(94.0)	(208.8)
Impact of the Act - one-time transition tax, net of foreign tax credits		0.0	13.1	653.7
Impact of the Act - deferred tax effects, net of valuation allowance		0.0	0.0	(136.7)
GILTI and foreign-derived intangible income (FDII), net of credits		54.7	95.4	0.0
Business realignment of intellectual property rights	$ (334.8)	(334.8)	(90.6)	0.0
Increases related to current year tax positions		20.1	9.0	32.0
Decreases related to prior year tax positions		(28.5)	(75.1)	(11.3)
Share-based compensation		(11.1)	(13.3)	(27.2)
Research and development tax credits		(40.8)	(27.5)	(17.0)
Intercompany prepaid tax asset amortization		0.0	5.2	7.4
Foreign exchange		(0.9)	4.6	(20.5)
Other		2.4	(2.6)	(0.5)
Change in valuation allowance		(28.6)	(9.9)	(20.5)
Total Income tax (benefit) provision		$ (420.2)	$ (151.4)	$ 481.9
Increase of basic net income per share due to release of prior year tax positions (in dollars per share)		$ 0.12	$ 0.32	$ 0.05
Increase of diluted net income per share due to release of prior year tax positions (in dollars per share)		$ 0.11	$ 0.30