Debt and Credit Facility - Debt and Credit Facility (Details) - USD ($) shares in Millions, $ in Millions	1 Months Ended				3 Months Ended		6 Months Ended		12 Months Ended
	Nov. 30, 2017	Jun. 30, 2017	Feb. 28, 2017	Feb. 28, 2015	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2015	May 31, 2018
Debt Instrument [Line Items]
Loss on settlement of convertible debt					$ 4.1	$ 0.0	$ 4.1	$ 13.8		$ 43.9	$ 50.6
Amount of unamortized debt discount of debentures					1,327.0		1,327.0		$ 1,372.9			$ 10.5
Debt issuance costs			$ 33.7		101.2		$ 101.2		40.1			$ 24.4
Equity portion of convertible debt issuance costs			12.5
Induced conversion of convertible debt expense			5.0
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)							8 years 4 months 16 days
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			2,040.0
Amount of unamortized debt discount of debentures					589.4		$ 589.4		616.3
Debt issuance costs					15.4		$ 15.4		16.1
Debt portion of convertible debt issuance costs			17.8
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)							6 years 4 months 16 days
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs				$ 1,690.0
Amount of unamortized debt discount of debentures					376.2		$ 376.2		400.3
Debt issuance costs					13.8		$ 13.8		14.8
Equity portion of convertible debt issuance costs				9.9
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)							18 years 4 months 16 days
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			567.7					111.3
Amount of unamortized debt discount of debentures					351.8		$ 351.8		356.3
Debt issuance costs				30.3	3.3		3.3		3.3
Debt portion of convertible debt issuance costs			3.4	20.4
2007 Junior Convertible Debt
Debt Instrument [Line Items]
Loss on settlement of convertible debt		$ 13.8							2.2
Long-term Debt | 2017 Junior Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs		111.3
Stock issued during period, value, conversion of convertible securities		119.3
Amount of unamortized debt discount of debentures					$ 55.1		$ 55.1
Long-term Debt | 2007 Junior Convertible Debt
Debt Instrument [Line Items]
Extinguishment of debt	$ 14.6	$ 111.3	$ 431.3	575.0				$ 111.3	$ 32.5
Stock issued during period, shares, conversion of convertible securities	0.5	3.2	12.0					3.2
Payment for debt extinguishment or debt prepayment cost	$ 41.0
Stock issued during period, value, conversion of convertible securities		$ 254.6	$ 862.7
Total consideration, extinguishment of debt		374.0	1,293.9	1,134.6
Total consideration, extinguishment of debt, liability component		56.3	188.0	238.3
Total consideration, extinguishment of debt, equity component		$ 321.1	$ 1,105.9	$ 896.3