Debt and Credit Facility (Details) - USD ($) $ in Millions	1 Months Ended				3 Months Ended			12 Months Ended
	Nov. 30, 2017	Jun. 30, 2017	Feb. 28, 2017	Feb. 28, 2015	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Debt Instrument [Line Items]
Debt issuance amortization			$ 33.7				$ 46.8	$ 40.1	$ 46.8
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
Shares issued to settle 2007 junior debt, value								298.5	862.7
Loss on settlement of convertible debt					$ 2.2	$ 13.8	43.9	16.0	43.9	$ 0.0	$ 50.6
Amount of unamortized debt discount of debentures							1,516.5	1,372.9	1,516.5
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs				$ 1,694.7
Debt issuance amortization				30.3			16.6	14.8	16.6
Debt portion of convertible debt issuance costs				20.4
Equity portion of convertible debt issuance costs				9.9
Amount of unamortized debt discount of debentures							446.6	400.3	446.6
2007 Junior Subordinated Convertible Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount								143.8
Extinguishment of debt, principal amount								$ 32.5
Shares converted								500,000
Debt issuance amortization							0.7	$ 0.0	0.7
Shares issued to settle convertible debt								3,700,000
Loss on settlement of convertible debt		$ 13.8						$ 2.2
Amount of unamortized debt discount of debentures							93.1	0.0	93.1
2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			567.7
Debt issuance amortization							3.4	3.3	3.4
Amount of unamortized debt discount of debentures							309.3	356.3	309.3
2017 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			2,043.6
Debt issuance amortization							17.6	16.1	17.6
Debt portion of convertible debt issuance costs			17.8
Amount of unamortized debt discount of debentures							$ 667.5	616.3	$ 667.5
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
Shares issued to settle convertible debt		3,200,000	11,997,924					3,200,000
Payment for Debt Extinguishment or Debt Prepayment Cost								$ 41.0
Shares issued to settle 2007 junior debt, value		$ 254.6	$ 862.7
Extinguishment of debt		374.0	1,293.9	1,134.6
Extinguishment of debt, liability component		56.3	188.0	238.3
Extinguishment of debt, equity component		321.1	$ 1,105.9	$ 896.3
Long-term Debt | 2017 Junior Subordinated 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