Debt - Summary of Conversion Features (Details) shares in Millions		1 Months Ended					3 Months Ended				12 Months Ended
	Mar. 27, 2020 USD ($)	Mar. 31, 2020 USD ($) shares	Nov. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) shares	Feb. 28, 2017 USD ($) shares	Feb. 28, 2015 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2020 USD ($) day	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($) shares	Mar. 31, 2017 USD ($)	Mar. 31, 2015 USD ($)
Debt Instrument [Line Items]
If-converted value in excess of principal											$ 1,000
Shares issued to settle convertible debt											351,800,000		$ 298,500,000
Loss on settlement of debt							$ 3,400,000	$ 1,900,000	$ 8,300,000	$ 4,100,000	5,400,000	$ 12,600,000	16,000,000.0	$ 43,900,000	$ 50,600,000
Gross long-term debt including current maturities		$ 10,593,300,000					10,593,300,000		11,659,300,000		10,593,300,000	11,659,300,000
Debt instrument, unamortized discount		1,043,200,000					1,043,200,000		1,268,700,000		1,043,200,000	1,268,700,000
Debt issuance costs, net		67,900,000			$ 33,700,000		67,900,000		83,600,000		$ 67,900,000	83,600,000
Debt portion of convertible debt issuance costs					3,400,000
Equity portion of convertible debt issuance costs					12,500,000
Induced conversion of convertible debt expense					5,000,000.0
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 10 months 24 days
Proceeds from debt, net of issuance costs					2,040,000,000.00
Debt portion of convertible debt issuance costs					17,800,000
2015 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)											4 years 10 months 24 days
Extinguishment of debt, principal amount		615,000,000.0
Loss on settlement of debt		3,400,000
Proceeds from debt, net of issuance costs						$ 1,690,000,000
Debt issuance costs, net						30,300,000
Debt portion of convertible debt issuance costs						20,400,000
Equity portion of convertible debt issuance costs						9,900,000
2015 Senior Convertible Debt | Long-term Debt
Debt Instrument [Line Items]
Extinguishment of debt, amount		$ 615,000,000.0
Shares issued to settle convertible debt (in shares) | shares		5.2
Shares issued to settle convertible debt		$ 351,800,000
Total consideration, extinguishment of debt		966,800,000
Total consideration, extinguishment of debt, liability component		460,400,000
Total consideration, extinguishment of debt, equity component		461,100,000
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 10 months 24 days
Proceeds from debt, net of issuance costs					567,700,000
2017 Junior Convertible Debt | Long-term Debt
Debt Instrument [Line Items]
Shares issued to settle convertible debt				$ 119,300,000
Proceeds from debt, net of issuance costs				111,300,000
Debt instrument, unamortized discount				55,100,000
2007 Junior Convertible Notes
Debt Instrument [Line Items]
Extinguishment of debt, principal amount													$ 32,500,000
Shares issued to settle convertible debt (in shares) | shares													0.5
Loss on settlement of debt				13,800,000									$ 2,200,000
2007 Junior Convertible Notes | Long-term Debt
Debt Instrument [Line Items]
Extinguishment of debt, amount			$ 14,600,000	$ 111,300,000	$ 431,300,000	575,000,000.0
Shares issued to settle convertible debt (in shares) | shares				3.2	12.0
Shares issued to settle convertible debt				$ 254,600,000	$ 862,700,000
Total consideration, extinguishment of debt				374,000,000.0	1,290,000,000	1,130,000,000
Total consideration, extinguishment of debt, liability component				56,300,000	188,000,000.0	238,300,000
Total consideration, extinguishment of debt, equity component				$ 321,100,000	$ 1,110,000,000	$ 896,300,000
Payment for debt extinguishment or debt prepayment cost													$ 41,000,000.0
Convertible Debt
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage											100.00%
Gross long-term debt including current maturities		3,866,300,000					3,866,300,000		4,481,300,000		$ 3,866,300,000	4,481,300,000
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
Line of Credit
Debt Instrument [Line Items]
Debt issuance costs, net		$ 3,900,000					3,900,000				$ 3,900,000
Bridge Loan Facility
Debt Instrument [Line Items]
Secured bridge agreement term	364 days	364 days
Gross long-term debt including current maturities		$ 615,000,000.0					615,000,000.0		0		615,000,000.0	0
Debt instrument, unamortized discount		3,100,000					3,100,000		0		3,100,000	0
Debt issuance costs, net	$ 6,200,000	$ 3,100,000					$ 3,100,000		$ 0		$ 3,100,000	$ 0