EMPLOYEE BENEFIT PLANS - Defined contribution plan (Details) - USD ($) |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|
| EMPLOYEE BENEFIT PLANS | |||
| Qualification age of employees for participation in the 401(k) savings plan | 21 years | ||
| Employer match of employee contributions of first 3% of eligible compensation (as a percent) | 100.00% | ||
| Percentage of eligible compensation, matched 100% by employer | 3.00% | ||
| Employer's match of employee's contributions of the next 5% of eligible compensation (as a percent) | 50.00% | ||
| Percentage of eligible compensation, matched 50% by employer | 5.00% | ||
| Expenses under the 401(k) saving plan | $ 3,463,000 | $ 3,353,000 | $ 2,696,000 |