Debt and Credit Facility (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	12 Months Ended
	Feb. 28, 2017	Feb. 28, 2015	Mar. 31, 2017	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Debt Instrument [Line Items]
Debt issuance amortization	$ 33,700			$ 46,800	$ 29,600
Equity portion of convertible debt issuance costs	12,500
Induced conversion of convertible debt expense	5,000
Shares issued to settle 2007 junior debt, value				862,651
Loss on settlement of convertible debt			$ 43,900	43,879	0	$ 50,631
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs		$ 1,694,700
Debt issuance amortization				16,600	18,400
Debt portion of convertible debt issuance costs	3,400
Equity portion of convertible debt issuance costs		9,900
2007 Junior Subordinated Convertible Notes
Debt Instrument [Line Items]
Debt issuance amortization				700	2,400
2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs	567,700
Debt issuance amortization		30,300		3,400	0
Debt portion of convertible debt issuance costs		20,400
2017 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs	2,043,600
Debt issuance amortization				$ 17,600	$ 0
Debt portion of convertible debt issuance costs	17,800
Long-term Debt | 2007 Junior Subordinated Convertible Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount	$ 431,300	575,000
Shares issued to settle convertible debt	11,997,924
Shares issued to settle 2007 junior debt, value	$ 862,700
Extinguishment of debt	1,293,900	1,134,600
Extinguishment of debt, liability component	188,000	238,300
Extinguishment of debt, equity component	$ 1,105,900	$ 896,300