Debt and Credit Facility - Components (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Mar. 31, 2019	May 31, 2018
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	$ (1,208,600)	$ (1,268,700)
Debt issuance cost	(75,100)	(83,600)
Line of Credit
Debt Instrument [Line Items]
Debt issuance cost	(1,750)
Senior Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures			$ (10,500)
Debt issuance cost			(24,400)
Senior Notes | 2023 Senior Secured Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(4,000)	(4,400)
Debt issuance cost	(8,900)	(10,200)
Senior Notes | 2021 Senior Secured Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(3,000)	(3,800)
Debt issuance cost	(6,800)	(8,800)
Senior Subordinated Convertible Debt | 2017 Senior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(533,400)	(561,900)
Debt issuance cost	(13,800)	(14,500)
Senior Subordinated Convertible Debt | 2015 Senior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(325,800)	(351,400)
Debt issuance cost	(11,900)	(12,800)
Junior Subordinated Debt | 2017 Junior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(342,400)	(347,200)
Debt issuance cost	(3,100)	(3,200)
Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Debt issuance cost	(14,600)	(14,700)
Term Loan Facility | Line of Credit
Debt Instrument [Line Items]
Debt issuance cost	$ (16,000)	$ (19,400)	$ (34,700)