CONVERTIBLE DEBT (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	May 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Convertible Debt	$ 300,000	$ 300,000	$ 0
Interest rate	0.625%	0.625%
Debt Conversion, Number of Shares of Common Stock Convertible per $1,000 of Principal Notes	4.8163
Convertible Preferred Stock, Terms of Conversion	1000
Debt Instrument, Convertible, Conversion Price	$ 207.63
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger	130.00%
Debt Instrument, Redemption Price, Percentage	100.00%
Nonconvertible Debt Borrowing Rate at the Date of Issuance	5.36%
Convertible Debt, Noncurrent	$ 238,400
Debt Instrument, Convertible, Carrying Amount of Equity Component	61,600
Interest Expense, Debt		$ 8,200
Interest Expense, Debt, Excluding Amortization		1,100
Amortization of convertible debt discount		6,385	0	$ 0
Amortization of debt issuance costs		1,032	245	$ 47
Debt Issuance Costs, Gross	9,300
Debt issuance costs, equity component	1,900
Long-term Debt, Fair Value		522,600
Debt Issuance Costs, Net [Abstract]
Gross carrying amount	300,000	300,000	0
Unamortized debt discount		55,202	0
Debt issuance costs	$ 7,400	6,599	0
Net carrying amount		238,199	$ 0
Convertible Notes Payable [Member]
Debt Instrument [Line Items]
Amortization of debt issuance costs		$ 700