Debt and Credit Facility - Debt Obligations (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Mar. 31, 2019	May 31, 2018
Debt Instrument [Line Items]
Long-term debt, gross	$ 11,086,300	$ 11,659,300
Less: Debt discount	(1,208,600)	(1,268,700)
Less: Debt issuance costs	(75,100)	(83,600)
Net long-term debt including current maturities	9,802,600	10,307,000
Less: Current maturities	(1,387,300)	(1,360,800)
Net long-term debt	8,415,300	8,946,200
Total Senior Secured Indebtedness
Debt Instrument [Line Items]
Long-term debt, gross	6,605,000	7,178,000
Line of Credit
Debt Instrument [Line Items]
Less: Debt issuance costs	(1,750)
Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt, gross	2,881,500	3,266,500
Less: Debt issuance costs	(14,600)	(14,700)
Line of Credit | Term Loan Facility
Debt Instrument [Line Items]
Long-term debt, gross	1,723,500	1,911,500
Less: Debt issuance costs	(16,000)	(19,400)	$ (34,700)
Senior Notes
Debt Instrument [Line Items]
Less: Debt discount			(10,500)
Less: Debt issuance costs			$ (24,400)
Senior Notes | 2023 Senior Secured Notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,000,000	1,000,000
Coupon Interest Rate	4.333%		4.333%
Effective Interest Rate	4.70%
Less: Debt discount	$ (4,000)	(4,400)
Less: Debt issuance costs	(8,900)	(10,200)
Senior Notes | 2021 Senior Secured Notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,000,000	1,000,000
Coupon Interest Rate	3.922%		3.922%
Effective Interest Rate	4.50%
Less: Debt discount	$ (3,000)	(3,800)
Less: Debt issuance costs	(6,800)	(8,800)
Convertible Debt
Debt Instrument [Line Items]
Long-term debt, gross	4,481,300	4,481,300
Senior Subordinated Convertible Debt | 2017 Senior Convertible Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 2,070,000	2,070,000
Coupon Interest Rate	1.625%
Effective Interest Rate	6.00%
Fair Value of Liability Component at Issuance	$ 1,396,300
Less: Debt discount	(533,400)	(561,900)
Less: Debt issuance costs	(13,800)	(14,500)
Senior Subordinated Convertible Debt | 2015 Senior Convertible Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,725,000	1,725,000
Coupon Interest Rate	1.625%
Effective Interest Rate	5.90%
Fair Value of Liability Component at Issuance	$ 1,160,100
Less: Debt discount	(325,800)	(351,400)
Less: Debt issuance costs	(11,900)	(12,800)
Junior Subordinated Debt | 2017 Junior Convertible Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 686,300	686,300
Coupon Interest Rate	2.25%
Effective Interest Rate	7.40%
Fair Value of Liability Component at Issuance	$ 321,100
Less: Debt discount	(342,400)	(347,200)
Less: Debt issuance costs	$ (3,100)	$ (3,200)