Debt and Credit Facility Debt Obligations (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017	Feb. 28, 2017	Feb. 28, 2015
Debt Instrument [Line Items]
Credit Facility		$ 0
Long-term debt, gross	$ 4,481,300	4,513,800
Total Convertible Debt	4,481,300	4,513,800
Less: Debt discount	(1,399,800)	(1,516,500)
Less: Debt issuance costs	(41,900)	(46,800)	$ (33,700)
Net long-term debt including current maturities	3,039,600	2,950,500
Current portion of long-term debt	0	(49,952)
Net long-term debt	3,039,623	2,900,524
December 2015 Amended Credit Facility with Maturity Dates of June 2018 and February 2020
Debt Instrument [Line Items]
Credit Facility	0	0
Less: Debt issuance costs	$ (6,700)	(8,500)
2017 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Coupon Interest Rate	1.625%
Effective Interest Rate	6.00%
Fair Value of Liability Component at Issuance (1)	$ 1,396,300
Long-term debt, gross	2,070,000	2,070,000
Less: Debt discount	(629,300)	(667,500)
Less: Debt issuance costs	$ (16,500)	(17,600)
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Coupon Interest Rate	1.625%
Effective Interest Rate	5.90%
Fair Value of Liability Component at Issuance (1)	$ 1,160,100
Long-term debt, gross	1,725,000	1,725,000
Less: Debt discount	(412,100)	(446,600)
Less: Debt issuance costs	$ (15,300)	(16,600)
2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Coupon Interest Rate	2.25%
Effective Interest Rate	7.40%
Fair Value of Liability Component at Issuance (1)	$ 321,100
Long-term debt, gross	686,300	575,000
Less: Debt discount	(358,400)	(309,300)
Less: Debt issuance costs	$ (3,400)	(3,400)		$ (30,300)
2007 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Coupon Interest Rate	2.125%
Effective Interest Rate	9.10%
Fair Value of Liability Component at Issuance (1)	$ 0
Long-term debt, gross	0	143,800
Less: Debt discount	0	(93,100)
Less: Debt issuance costs	$ 0	$ (700)