Debt and Credit Facility - Debt Obligations (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	May 31, 2018
Debt Instrument [Line Items]
Long-term debt, gross	$ 10,829,300		$ 11,659,300
Less: Debt discount	(1,177,900)		(1,268,700)
Less: Debt issuance costs	(70,900)		(83,600)
Net long-term debt including current maturities	9,580,500		10,307,000
Less: Current maturities	(1,400,800)		(1,360,800)
Net long-term debt	8,179,700		8,946,200
Total Senior Secured Indebtedness
Debt Instrument [Line Items]
Long-term debt, gross	6,348,000		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	2,624,500		3,266,500
Less: Debt issuance costs	(13,600)		(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	$ (15,300)		(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]
Coupon Interest Rate	4.333%			4.333%
Effective Interest Rate	4.70%
Long-term debt, gross	$ 1,000,000		1,000,000
Less: Debt discount	(3,700)		(4,400)
Less: Debt issuance costs	$ (8,300)		(10,200)
Senior Notes | 2021 Senior Secured Notes
Debt Instrument [Line Items]
Coupon Interest Rate	3.922%			3.922%
Effective Interest Rate	4.50%
Long-term debt, gross	$ 1,000,000		1,000,000
Less: Debt discount	(2,600)		(3,800)
Less: Debt issuance costs	(5,800)		(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]
Coupon Interest Rate	1.625%
Effective Interest Rate	6.00%
Fair Value of Liability Component at Issuance	$ 1,396,300
Long-term debt, gross	2,070,000		2,070,000
Less: Debt discount	(518,900)		(561,900)
Less: Debt issuance costs	$ (13,400)		(14,500)
Senior Subordinated Convertible Debt | 2015 Senior Convertible Debt
Debt Instrument [Line Items]
Coupon Interest Rate	1.625%
Effective Interest Rate	5.90%
Fair Value of Liability Component at Issuance	$ 1,160,100
Long-term debt, gross	1,725,000		1,725,000
Less: Debt discount	(312,800)		(351,400)
Less: Debt issuance costs	$ (11,400)		(12,800)
Junior Subordinated Debt | 2017 Junior Convertible Debt
Debt Instrument [Line Items]
Coupon Interest Rate	2.25%
Effective Interest Rate	7.40%
Fair Value of Liability Component at Issuance	$ 321,100
Long-term debt, gross	686,300		686,300
Less: Debt discount	(339,900)		(347,200)
Less: Debt issuance costs	$ (3,100)		$ (3,200)