Debt and Credit Facility (Details) - USD ($) $ in Thousands, shares in Millions	1 Months Ended				3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Nov. 30, 2017	Jun. 30, 2017	Feb. 28, 2017	Feb. 28, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2017	Mar. 31, 2015
Debt Instrument [Line Items]
Loss on settlement of convertible debt					$ 2,140	$ 0		$ 15,966	$ 0	$ 43,900	$ 50,600
Amount of unamortized debt discount of debentures					1,399,800		$ 1,399,800	1,399,800		1,516,500
Debt issuance costs			$ 33,700		41,900		41,900	41,900		46,800
Equity portion of convertible debt issuance costs			12,500
Induced conversion of convertible debt expense			5,000
2017 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			2,043,600
Amount of unamortized debt discount of debentures					629,300		629,300	629,300		667,500
Debt issuance costs					16,500		16,500	16,500		17,600
Debt portion of convertible debt issuance costs			17,800
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs				$ 1,694,700
Amount of unamortized debt discount of debentures					412,100		412,100	412,100		446,600
Debt issuance costs					15,300		15,300	15,300		16,600
Debt portion of convertible debt issuance costs			3,400
Equity portion of convertible debt issuance costs				9,900
2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			567,700
Amount of unamortized debt discount of debentures					358,400		358,400	358,400		309,300
Debt issuance costs				30,300	3,400		3,400	3,400		3,400
Debt portion of convertible debt issuance costs				20,400
2007 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Loss on settlement of convertible debt		$ 13,800					2,100
Amount of unamortized debt discount of debentures					0		0	0		93,100
Debt issuance costs					0		0	0		$ 700
Long-term Debt | 2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs		111,300
Stock issued during period, value, conversion of convertible securities		119,300
Amount of unamortized debt discount of debentures					$ 55,100		55,100	55,100
Long-term Debt | 2007 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Extinguishment of debt	$ 14,600	111,300	431,300	575,000			$ 32,400	$ 111,300
Total consideration, extinguishment of debt		374,000	1,293,900	1,134,600
Stock issued during period, value, conversion of convertible securities		$ 254,600	$ 862,700
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	$ 41,000
Total consideration, extinguishment of debt, liability component		$ 56,300	$ 188,000	238,300
Total consideration, extinguishment of debt, equity component		$ 321,100	$ 1,105,900	$ 896,300