Debt and Credit Facility - Debt and Credit Facility (Details) - USD ($) shares in Millions, $ in Millions	1 Months Ended				3 Months Ended					12 Months Ended
	Nov. 30, 2017	Jun. 30, 2017	Feb. 28, 2017	Feb. 28, 2015	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2015	May 31, 2018
Debt Instrument [Line Items]
Loss on settlement of debt					$ 8.3	$ 0.2	$ 4.1	$ 2.1	$ 13.8	$ 12.6	$ 16.0	$ 43.9	$ 50.6
Shares issued to settle 2007 junior debt, value											298.5	862.7
Amount of unamortized debt discount of debentures					1,268.7					1,268.7	1,372.9			$ 10.5
Debt issuance amortization			$ 33.7		83.6					83.6	40.1			$ 24.4
Debt portion of convertible debt issuance costs			3.4
Equity portion of convertible debt issuance costs			12.5
Induced conversion of convertible debt expense			5.0
2015 Senior Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs				$ 1,690.0
Amount of unamortized debt discount of debentures					351.4					351.4	400.3
Debt issuance amortization				30.3	12.8					12.8	14.8
Debt portion of convertible debt issuance costs				20.4
Equity portion of convertible debt issuance costs				9.9
2007 Junior Subordinated Convertible Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount											143.8	$ 431.3
Extinguishment of debt, principal amount											32.5
Shares converted (in shares)	0.5
Loss on settlement of debt		$ 13.8									$ 2.2
Shares issued to settle convertible debt (in shares)											3.7	12.0
2017 Junior Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			567.7
Amount of unamortized debt discount of debentures					347.2					347.2	$ 356.3
Debt issuance amortization					3.2					3.2	3.3
2017 Senior Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			2,040.0
Amount of unamortized debt discount of debentures					561.9					561.9	616.3
Debt issuance amortization					14.5					14.5	16.1
Debt portion of convertible debt issuance costs			17.8
Long-term Debt | 2007 Junior Subordinated Convertible Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount	$ 14.6	111.3	431.3	575.0							$ 111.3
Redemption costs	$ 41.0
Shares issued to settle 2007 junior debt, value		$ 254.6	$ 862.7
Shares issued to settle convertible debt (in shares)		3.2	12.0								3.2
Extinguishment of debt, liability component		$ 56.3	$ 188.0	238.3
Extinguishment of debt		374.0	1,290.0	1,130.0
Extinguishment of debt, equity component		321.1	$ 1,110.0	$ 896.3
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
Shares issued to settle 2007 junior debt, value		$ 119.3
Amount of unamortized debt discount of debentures					$ 55.1					$ 55.1