Debt and Credit Facility (Details) - USD ($) $ in Thousands, shares in Millions	1 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Feb. 28, 2017	Feb. 28, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2017	Mar. 31, 2015
Debt Instrument [Line Items]
Loss on settlement of convertible debt				$ 0	$ 0	$ (13,826)	$ 0	$ (43,900)	$ (50,600)
Amount of unamortized debt discount of debentures				1,437,700		1,437,700		1,516,500
Debt issuance costs		$ 33,700		43,000		43,000		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				642,200		642,200		667,500
Debt issuance costs				16,900		16,900		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				423,800		423,800		446,600
Debt issuance costs				15,700		15,700		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				360,500		360,500		309,300
Debt issuance costs			30,300	3,300		3,300		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
Amount of unamortized debt discount of debentures				11,200		11,200		93,100
Debt issuance costs				100		100		$ 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
Long-term Debt | 2007 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Extinguishment of debt	111,300	431,300	575,000			$ 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	3.2	12.0		0.5		3.2
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