DEBT - 2017 Credit Facility (Details) - USD ($)		12 Months Ended
	May 16, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 08, 2018	Apr. 04, 2017	Sep. 30, 2014
Line of Credit Facility [Line Items]
Debt, gross		$ 297,608,000	$ 281,125,000
Principal payments of convertible senior notes		126,892,000	0	$ 0
Interest paid	$ 1,100,000	20,972,000	8,426,000	3,883,000
Write-off of deferred financing costs		0	0	340,000
Loss on extinguishment of debt		11,676,000	0	0
Non-cash interest expense		4,129,000	$ 7,252,000	6,294,000
Settlement of capped call transactions		2,118,000
$135 million Term Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity						$ 135,000,000
Convertible Senior Notes | Convertible Notes
Line of Credit Facility [Line Items]
Debt, gross		$ 22,608,000					$ 149,500,000
Effective rate (percent)		7.40%	6.90%				2.50%
Principal payments of convertible senior notes	126,900,000
Third party fee	700,000
Write off of debt discount	5,900,000
Write-off of deferred financing costs	$ 900,000
Loss on extinguishment of debt		$ 7,500,000
Interest expense recognized on debt agreement		4,900,000	$ 10,300,000	10,000,000
Non-cash interest expense		2,700,000	5,600,000	5,400,000
Amortization of financing costs		400,000	900,000	$ 900,000
Revolving Loan Facility 2017
Line of Credit Facility [Line Items]
Maximum borrowing capacity						$ 50,000,000
Credit Facility 2017
Line of Credit Facility [Line Items]
Unamortized debt issuance expense					$ 3,600,000
Additional Paid-In Capital (APIC)
Line of Credit Facility [Line Items]
Settlement of capped call transactions		2,118,000
Level 2
Line of Credit Facility [Line Items]
Convertible debt, fair value		$ 22,300,000	$ 141,800,000