SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Financial Instruments Carried at Fair Value (Details) - Estimate of fair value - Money market funds - USD ($) $ in Thousands |
Dec. 31, 2016 |
Dec. 31, 2015 |
|---|---|---|
| Fair value measurements | ||
| Cash and cash equivalents | $ 125,515 | $ 94,912 |
| Quoted Prices in active markets (Level 1) | ||
| Fair value measurements | ||
| Cash and cash equivalents | $ 125,515 | $ 94,912 |