CONVERTIBLE DEBT - Issuances and Settlements (Details) - USD ($) shares in Millions	1 Months Ended			3 Months Ended	12 Months Ended
	Mar. 31, 2019	Jun. 30, 2018	Jan. 31, 2018	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Net proceeds from issuance					$ 729,477,000	$ 896,430,000	$ 0
Value of stock after conversion of convertible notes					182,477,000
Settlement of equity component of convertible notes					(300,768,000)	0	0
Loss on extinguishment of debt					10,558,000	$ 0	$ 0
2025 Convertible notes
Debt Instrument [Line Items]
Stock issued in conversion of convertible notes (in shares)	2.2
Value of stock after conversion of convertible notes	$ 182,400,000
Repayments of convertible debt in cash and by issuance of shares	676,500,000
Repayments of debt	493,400,000
Consideration settled on liability component of 2025 notes	375,100,000			$ 375,100,000
Settlement of equity component of convertible notes				300,800,000
Accrued interest on notes	600,000
Loss on extinguishment of debt					$ 10,600,000
2027 Convertible notes
Debt Instrument [Line Items]
Face amount	747,500,000			$ 747,500,000
Interest rate (as a percent)					0.375%
Net proceeds from issuance	729,500,000
2027 Convertible notes | 2025 Convertible notes
Debt Instrument [Line Items]
Repayments of convertible debt	$ 494,000,000.0
2025 Convertible notes
Debt Instrument [Line Items]
Face amount		$ 218,500,000	$ 690,000,000.0
Interest rate (as a percent)					1.00%
Net proceeds from issuance		$ 225,300,000	$ 671,100,000