Debt - Summary of Conversion Features (Details) shares in Millions	1 Months Ended			3 Months Ended			6 Months Ended
	Aug. 31, 2020 USD ($) shares	Jun. 30, 2020 USD ($) shares	Mar. 31, 2020 USD ($) shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) day	Sep. 30, 2019 USD ($)
Debt Instrument [Line Items]
If-converted value in excess of principal							$ 1,000
Gain (loss) on settlement of debt	$ (45,100,000)	$ (21,500,000)		$ (45,100,000)		$ (100,000)	$ (71,900,000)	$ (2,000,000.0)
Shares issued to settle convertible debt				$ 768,700,000	$ 651,500,000
2017 Senior Convertible Debt
Debt Instrument [Line Items]
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)							6 years 4 months 24 days
Extinguishment of debt, principal amount	381,800,000	643,900,000
2017 Senior Convertible Debt | Long-term Debt
Debt Instrument [Line Items]
Extinguishment of debt, amount	$ 381,800,000	$ 643,900,000
Shares issued to settle convertible debt (in shares) | shares	2.1	2.5
Shares issued to settle convertible debt	$ 221,100,000	$ 246,400,000
Total consideration, extinguishment of debt	602,900,000	890,300,000
Total consideration, extinguishment of debt, liability component	299,000,000.0	481,000,000.0
Total consideration, extinguishment of debt, equity component	292,200,000	390,900,000
2015 Senior Convertible Debt
Debt Instrument [Line Items]
Threshold percentage of stock price trigger							130.00%
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)							4 years 4 months 24 days
Extinguishment of debt, principal amount	414,300,000	383,300,000	$ 615,000,000.0
2015 Senior Convertible Debt | Long-term Debt
Debt Instrument [Line Items]
Extinguishment of debt, amount	$ 414,300,000	$ 383,300,000	$ 615,000,000.0
Shares issued to settle convertible debt (in shares) | shares	5.2	4.1	5.2
Shares issued to settle convertible debt	$ 547,600,000	$ 405,100,000	$ 351,800,000
Total consideration, extinguishment of debt	961,800,000	788,400,000	$ 966,800,000
Total consideration, extinguishment of debt, liability component	351,700,000	314,400,000
Total consideration, extinguishment of debt, equity component	$ 592,300,000	464,400,000
2017 Junior Convertible Debt
Debt Instrument [Line Items]
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)							16 years 4 months 24 days
Convertible Debt
Debt Instrument [Line Items]
Debt instrument, redemption price (percent)							100.00%
Convertible Debt | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Threshold percentage of stock price trigger							130.00%
Threshold trading days | day							20
Threshold consecutive trading days | day							30
Convertible Debt | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Threshold percentage of stock price trigger							98.00%
Threshold trading days | day							5
Threshold consecutive trading days | day							10
Senior Indebtedness | 2017 Senior Convertible Debt
Debt Instrument [Line Items]
Debt instrument, face amount		643,900,000			$ 643,900,000
Senior Indebtedness | 2015 Senior Convertible Debt
Debt Instrument [Line Items]
Extinguishment of debt, amount		383,300,000
Bridge Loan Facility
Debt Instrument [Line Items]
Extinguishment of debt, amount		615,000,000.0
Gain (loss) on settlement of debt		$ (5,300,000)