DEBT - Convertible Subordinated Notes (Details) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2013 USD ($) item $ / shares $ / item shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) $ / shares $ / item shares
Debt
Settlement amount for purchase of notes		$ 0	$ 11,570	$ 0
Amount received from capped call option		$ 0	578	$ 0
2023 Notes | Convertible subordinated notes
Debt
Loan amount	$ 287,500
Proceeds from issuance of notes payable, net of debt issuance costs	$ 281,200
Interest rate (as a percent)	2.125%
Initial conversion, shares per $1,000 principal amount | shares | shares	35.9903
Adjusted conversion, shares per $1,000 principal amount | shares				50.5818
Conversion price of convertible notes into common stock (in dollars per share) | $ / shares	$ 27.79			$ 19.77
Portion of debt retired, face value			14,100
Portion of debt retired, carrying value			13,900
Settlement amount for purchase of notes			11,600
2023 Notes | Convertible subordinated notes | Other (expense) income
Debt
Gain (Loss) on Repurchase of Debt Instrument			2,300
2023 Notes | Convertible subordinated notes | Privately-negotiated capped call option
Debt
Payments for capped call options	$ 36,800
Number of derivative instruments purchased | item	2
Strike price for the underlying number of shares (in dollars per share) | $ / shares	$ 27.79			$ 19.77
Cap price for the underlying number of shares (in dollars per share) | $ / item	38.00			27.04
Amount received from capped call option			$ 600
2023 Notes | Convertible subordinated notes | Stock prices below $27.79 per share | Minimum
Debt
Net shares settlement payable to the entity | shares	0
2023 Notes | Convertible subordinated notes | Stock prices above $38.00 per share | Maximum
Debt
Net shares settlement payable to the entity | shares	2,779,659