Income tax (benefit) provision from continuing operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Results of Operations, Income before Income Taxes [Abstract]
U.S.	$ (127.3)	$ (279.3)	$ (75.5)
Foreign	864.6	369.1	356.8
Income before income taxes	737.3	89.8	281.3
Current expense (benefit):
U.S. Federal	369.4	21.3	(4.0)
State	0.5	1.0	(0.2)
Foreign	60.8	23.8	22.0
Total current	430.7	46.1	17.8
Deferred expense (benefit):
U.S. Federal	82.5	(114.7)	(42.2)
State	0.1	(5.4)	(2.0)
Foreign	(31.4)	(6.8)	(16.2)
Total deferred	51.2	(126.9)	(60.4)
Total Income tax provision (benefit)	481.9	(80.8)	(42.6)
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Computed expected income tax provision	232.6	31.4	98.4
Foreign income taxed at lower than the federal rate	(208.8)	(105.0)	(120.1)
Impact of the Act - one-time transition tax, net of foreign tax credits	653.7	0.0	0.0
Impact of the Act - deferred tax effects, net of valuation allowance	(136.7)	0.0	0.0
Impact of the Act - deferred tax effects, net of valuation allowance	32.0	53.7	14.5
Decreases related to prior year tax positions (1)	(11.3)	(36.3)	(12.1)
Decreases related to prior year tax positions (1)	(27.2)	(25.0)	0.0
Research and development tax credits - current year	(17.0)	(12.8)	(13.5)
Research and development tax credits - current year	0.0	0.0	(2.5)
Intercompany prepaid tax asset amortization	7.4	7.9	(15.5)
Withholding taxes	1.4	5.6	6.0
Foreign exchange	(20.5)	(1.7)	5.6
Other	(3.2)	(0.4)	(0.9)
Change in valuation allowance	(20.5)	1.8	(2.5)
Income tax provision (benefit)	$ 481.9	$ (80.8)	$ (42.6)
Increase of basic net income per share due to release of prior year tax positions	$ 0.05	$ 0.17	$ 0.06
Increase of diluted net income per share due to release of prior year tax positions	$ 0.05	$ 0.15	$ 0.06
Income tax holiday, aggregate dollar amount	$ 6.2	$ 13.2	$ 9.4
Income tax holiday, benefits per share, basic	$ 0.03	$ 0.06	$ 0.05
Income tax holiday, benefits per share, diluted	$ 0.02	$ 0.06	$ 0.04