STOCKHOLDERS' EQUITY - Common Stock Reserved for Future Issuance (Details) - shares |
Dec. 31, 2016 |
Dec. 31, 2015 |
|---|---|---|
| Class of Stock [Line Items] | ||
| Total reserved shares of common stock for future issuance | 17,277,802 | |
| Employee Stock Options and Restricted Stock Units | ||
| Class of Stock [Line Items] | ||
| Options and Restricted Stock Units issued and outstanding | 10,585,444 | |
| 2014 Equity Incentive Plan | ||
| Class of Stock [Line Items] | ||
| Shares reserved for issuance under share-based compensation plan | 5,116,753 | 5,017,525 |
| Employee Stock Purchase Plan | ESPP | ||
| Class of Stock [Line Items] | ||
| Shares reserved for issuance under share-based compensation plan | 1,575,605 |