SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair value Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair value measurements
Long-term debt, carrying value	$ 156,800	$ 32,900
Significant Other Observable Inputs (Level 2)
Fair value measurements
Marketable securities, fair value	2,000
Long-term Debt, Fair Value	159,900	32,900
Recurring basis | Total
Fair value measurements
Total assets measured at fair value	68,636	76,640
Recurring basis | Certificate of deposit | Total
Fair value measurements
Cash and cash equivalents	391	377
Recurring basis | Money market funds | Total
Fair value measurements
Cash and cash equivalents	29,027	76,263
Recurring basis | Short Term Investments | Total
Fair value measurements
Investments, Fair Value Disclosure	9,237
Recurring basis | Commercial Paper | Total
Fair value measurements
Cash and cash equivalents	29,981
Recurring basis | Quoted Prices in Active Markets (Level 1)
Fair value measurements
Total assets measured at fair value	30,778	76,263
Recurring basis | Quoted Prices in Active Markets (Level 1) | Money market funds
Fair value measurements
Cash and cash equivalents	29,027	76,263
Recurring basis | Quoted Prices in Active Markets (Level 1) | Short Term Investments
Fair value measurements
Investments, Fair Value Disclosure	1,751
Recurring basis | Significant Other Observable Inputs (Level 2)
Fair value measurements
Total assets measured at fair value	37,858	377
Recurring basis | Significant Other Observable Inputs (Level 2) | Certificate of deposit
Fair value measurements
Cash and cash equivalents	391	$ 377
Recurring basis | Significant Other Observable Inputs (Level 2) | Short Term Investments
Fair value measurements
Investments, Fair Value Disclosure	7,486
Recurring basis | Significant Other Observable Inputs (Level 2) | Commercial Paper
Fair value measurements
Cash and cash equivalents	$ 29,981