EMPLOYEE BENEFIT PLANS - Level 3 Financial Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Qualified Pension Plan | Level 3 | Group deposit administration annuity contract | Contract Value
Employee benefit plans
Fair value	$ 75,052
Qualified Pension Plan | Level 3 | Group deposit administration annuity contract | Contract Value | Minimum
Employee benefit plans
Contract rate (as a percent)	5.01%
Qualified Pension Plan | Level 3 | Group deposit administration annuity contract | Contract Value | Maximum
Employee benefit plans
Contract rate (as a percent)	5.08%
Group life insurance plan | Money market fund
Employee benefit plans
Fair value	$ 4,610	$ 4,854
Group life insurance plan | Money market fund | Level 1
Employee benefit plans
Fair value	4,610	4,854
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	304,822	253,955	$ 260,926
Qualified Plan | Qualified Pension Plan | Level 1
Employee benefit plans
Fair value	229,770	175,248
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	75,052	78,707	$ 74,886
Qualified Plan | Qualified Pension Plan | Group deposit administration annuity contract
Employee benefit plans
Fair value	75,052	78,707
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	$ 75,052	$ 78,707