EMPLOYEE BENEFIT PLANS - Level 3 Financial Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Qualified Pension Plan | Level 3 | Group deposit administration annuity contract | Contract Value | Minimum
Employee benefit plans
Contract rate (as a percent)	5.06%
Qualified Pension Plan | Level 3 | Group deposit administration annuity contract | Contract Value | Maximum
Employee benefit plans
Contract rate (as a percent)	5.14%
Group life insurance plan | Money market fund
Employee benefit plans
Fair value	$ 4,854	$ 5,104
Group life insurance plan | Money market fund | Level 1
Employee benefit plans
Fair value	4,854	5,104
Group life insurance plan | Money market fund | Level 2
Employee benefit plans
Fair value	0	0
Group life insurance plan | Money market fund | Level 3
Employee benefit plans
Fair value	0	0
Qualified Plan | Qualified Pension Plan
Employee benefit plans
Fair value	253,955	260,926	$ 201,843
Qualified Plan | Qualified Pension Plan | Level 1
Employee benefit plans
Fair value	175,248	186,040
Qualified Plan | Qualified Pension Plan | Level 2
Employee benefit plans
Fair value	0	0
Qualified Plan | Qualified Pension Plan | Level 3
Employee benefit plans
Fair value	78,707	74,886	$ 71,226
Qualified Plan | Qualified Pension Plan | Group deposit administration annuity contract
Employee benefit plans
Fair value	78,707	74,886
Qualified Plan | Qualified Pension Plan | Group deposit administration annuity contract | Level 1
Employee benefit plans
Fair value	0	0
Qualified Plan | Qualified Pension Plan | Group deposit administration annuity contract | Level 2
Employee benefit plans
Fair value	0	0
Qualified Plan | Qualified Pension Plan | Group deposit administration annuity contract | Level 3
Employee benefit plans
Fair value	$ 78,707	$ 74,886