1.625% Senior Subordinated Convertible Debentures (Details)	3 Months Ended		6 Months Ended
	Sep. 30, 2016 USD ($) $ / shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) $ / shares	Sep. 30, 2015 USD ($)	Mar. 31, 2016 USD ($)	Feb. 28, 2015 USD ($)
Debt Instrument [Line Items]
Amortization of debt discount on convertible debentures			$ 24,413,000	$ 23,746,000
Senior Subordinated Convertible Debenture Due 2025
Debt Instrument [Line Items]
Principal amount of senior subordinated convertible debentures						$ 1,725,000,000.0
Interest rate of senior subordinated convertible debentures	1.625%		1.625%
Due date of senior subordinated convertible debentures			Feb. 15, 2025
Conversion rate, in shares, of senior subordinated convertible debt (in shares per $1,000)			14.5654
Principal amount of senior subordinated convertible debentures used as conversion unit			$ 1,000
Convertible debt instrument initial conversion price per share | $ / shares	$ 68.66		$ 68.66
Debt instrument, convertible, conversion ratio			1534.37%
Debt instrument, convertible, conversion price | $ / shares	$ 65.17		$ 65.17
Debt instrument, convertible, conversion ratio increase option, initial ratio at issuance			7.2827
Debt instrument, convertible, conversion ratio increase option, adjusted for dividends			7.6718
Debt instrument, convertible, conversion ratio maximum conversion rate including increase option			20.3915
Debt instrument, convertible, conversion ratio maximum conversion rate including increase option, adjusted for dividends paid			21.4811
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			$ 1,694,700,000
Senior subordinated convertible debenture debt issuance costs			30,300,000
Debt portion of convertible debt issuance costs			20,400,000
Equity portion of convertible debt issuance costs			9,900,000
Carrying value of equity component of the senior subordinated convertible debentures that were bifurcated into a liability and equity component	$ 564,900,000		564,900,000		$ 564,900,000
Estimated fair value of the liability component of the senior subordinated convertible debentures as of the issuance date	1,160,100,000		1,160,100,000
Debt discount of estimated fair value of liability component of senior subordinated convertible debentures at the issuance date	564,900,000		564,900,000
Amount of unamortized debt discount of debentures	468,700,000		$ 468,700,000		$ 490,300,000
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)			8 years 4 months 18 days
Amortization of debt discount on convertible debentures	10,800,000	$ 10,600,000	$ 21,500,000	21,100,000
Recognized amount of interest expense related to coupon on senior subordinated convertible debentures	$ 7,000,000	$ 7,000,000	$ 14,000,000	$ 14,000,000
Effective interest rate of senior convertible debentures			5.90%