EMPLOYEE BENEFIT PLANS - Plan Assets at Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Group life insurance plan | Money market fund
Fair value hierarchy of Plan's assets
Total investments	$ 5,104	$ 5,104
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	5,104
Fair value of plan assets at end of year	4,854	5,104
Group life insurance plan | Level 1 | Money market fund
Fair value hierarchy of Plan's assets
Total investments	5,104	5,104
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	5,104
Fair value of plan assets at end of year	4,854	5,104
Group life insurance plan | Level 2 | Money market fund
Fair value hierarchy of Plan's assets
Total investments	0	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Group life insurance plan | Level 3 | Money market fund
Fair value hierarchy of Plan's assets
Total investments	0	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Qualified Plan | Qualified Pension Plan
Fair value hierarchy of Plan's assets
Total investments	260,926	201,843
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	260,926	201,843
Fair value of plan assets at end of year	253,955	260,926
Qualified Plan | Qualified Pension Plan | Cash
Fair value hierarchy of Plan's assets
Total investments	39,897	39,897
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	39,897
Fair value of plan assets at end of year	1,225	39,897
Qualified Plan | Qualified Pension Plan | Equity securities
Fair value hierarchy of Plan's assets
Total investments	146,143	146,143
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	146,143
Fair value of plan assets at end of year	158,823	146,143
Qualified Plan | Qualified Pension Plan | Fixed income
Fair value hierarchy of Plan's assets
Total investments	15,200
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at end of year	15,200
Qualified Plan | Qualified Pension Plan | Group deposit administration annuity contract
Fair value hierarchy of Plan's assets
Total investments	74,886	74,886
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	74,886
Fair value of plan assets at end of year	78,707	74,886
Qualified Plan | Qualified Pension Plan | Level 1
Fair value hierarchy of Plan's assets
Total investments	186,040	186,040
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	186,040
Fair value of plan assets at end of year	175,248	186,040
Qualified Plan | Qualified Pension Plan | Level 1 | Cash
Fair value hierarchy of Plan's assets
Total investments	39,897	39,897
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	39,897
Fair value of plan assets at end of year	1,225	39,897
Qualified Plan | Qualified Pension Plan | Level 1 | Equity securities
Fair value hierarchy of Plan's assets
Total investments	146,143	146,143
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	146,143
Fair value of plan assets at end of year	158,823	146,143
Qualified Plan | Qualified Pension Plan | Level 1 | Fixed income
Fair value hierarchy of Plan's assets
Total investments	15,200
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at end of year	15,200
Qualified Plan | Qualified Pension Plan | Level 1 | Group deposit administration annuity contract
Fair value hierarchy of Plan's assets
Total investments	0	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Qualified Plan | Qualified Pension Plan | Level 2
Fair value hierarchy of Plan's assets
Total investments	0	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Qualified Plan | Qualified Pension Plan | Level 2 | Cash
Fair value hierarchy of Plan's assets
Total investments	0	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Qualified Plan | Qualified Pension Plan | Level 2 | Equity securities
Fair value hierarchy of Plan's assets
Total investments	0	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Qualified Plan | Qualified Pension Plan | Level 2 | Fixed income
Fair value hierarchy of Plan's assets
Total investments	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at end of year	0
Qualified Plan | Qualified Pension Plan | Level 2 | Group deposit administration annuity contract
Fair value hierarchy of Plan's assets
Total investments	0	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Qualified Plan | Qualified Pension Plan | Level 3
Fair value hierarchy of Plan's assets
Total investments	74,886	71,226
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	74,886	71,226
Interest income	3,821	3,660
Transfers from collective short-term investments fund	0	0
Transfers to collective short-term investments fund	0	0
Fair value of plan assets at end of year	78,707	74,886
Qualified Plan | Qualified Pension Plan | Level 3 | Cash
Fair value hierarchy of Plan's assets
Total investments	0	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Qualified Plan | Qualified Pension Plan | Level 3 | Equity securities
Fair value hierarchy of Plan's assets
Total investments	0	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Qualified Plan | Qualified Pension Plan | Level 3 | Fixed income
Fair value hierarchy of Plan's assets
Total investments	0
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at end of year	0
Qualified Plan | Qualified Pension Plan | Level 3 | Group deposit administration annuity contract
Fair value hierarchy of Plan's assets
Total investments	74,886	74,886
Reconciliation of beginning and ending balances for fair value measurements for which significant unobservable inputs (level 3) are used
Fair value of plan assets at beginning of year	74,886
Fair value of plan assets at end of year	$ 78,707	$ 74,886