Debt - Settlement of debt (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended			6 Months Ended
	Aug. 31, 2021	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Debt Instrument [Line Items]
Cash Paid						$ 1,000.0	$ 0.0
Value of Shares Issued			$ 399.2	$ 768.7	$ 651.5
Loss on settlement of debt			$ (85.2)	$ (45.1)		$ (85.5)	$ (71.9)
3.922% 2021 Notes | Senior Indebtedness
Debt Instrument [Line Items]
Principal Amount Settled		$ 1,000.0
Cash Paid		1,000.0
Total		1,000.0
Loss on settlement of debt		$ 0.3
2015 Senior Convertible Debt | Senior Convertible Debt
Debt Instrument [Line Items]
Principal Amount Settled	$ 70.4
Cash Paid	70.4
Value of Shares Issued	159.9
Total	230.3
Fair Value Settled	71.0
Equity Component	158.9
Loss on settlement of debt	10.6
2017 Senior Convertible Debt | Senior Convertible Debt
Debt Instrument [Line Items]
Principal Amount Settled	100.7
Cash Paid	100.7
Value of Shares Issued	123.5
Total	224.2
Fair Value Settled	100.0
Equity Component	113.0
Loss on settlement of debt	31.5
2017 Junior Convertible Debt | Junior Convertible Debt
Debt Instrument [Line Items]
Principal Amount Settled	92.5
Cash Paid	92.5
Value of Shares Issued	115.8
Total	208.3
Fair Value Settled	87.7
Equity Component	116.6
Loss on settlement of debt	$ 43.1
0.983% 2024 Notes | Senior Indebtedness
Debt Instrument [Line Items]
Debt instrument, redemption price (percent)						101.00%