EMPLOYEE BENEFIT PLANS - Plan Assets at Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Group life insurance plan | Money Market Funds
Fair value hierarchy of Plan's assets
Total investments	$ 5,362	$ 5,362
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	5,362
Fair value of plan assets at end of year	5,104	5,362
Group life insurance plan | Level 1 | Money Market Funds
Fair value hierarchy of Plan's assets
Total investments	5,362	5,362
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	5,362
Fair value of plan assets at end of year	5,104	5,362
Group life insurance plan | Level 2 | Money Market Funds
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 Funds
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	201,843	196,042
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	201,843	196,042
Fair value of plan assets at end of year	260,926	201,843
Qualified Plan | Qualified Pension Plan | Collective short-term investment fund
Fair value hierarchy of Plan's assets
Total investments	$ 4,175	$ 4,175
Redemption Notice Period	1 day	1 day
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	$ 4,175
Fair value of plan assets at end of year	39,897	$ 4,175
Qualified Plan | Qualified Pension Plan | Equity index funds
Fair value hierarchy of Plan's assets
Total investments	126,442	126,442
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	126,442
Fair value of plan assets at end of year	146,143	126,442
Qualified Plan | Qualified Pension Plan | Group deposit administration annuity contract
Fair value hierarchy of Plan's assets
Total investments	71,226	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	71,226
Fair value of plan assets at end of year	74,886	71,226
Qualified Plan | Qualified Pension Plan | Equity securities: Collective Russell 3000 index fund
Fair value hierarchy of Plan's assets
Total investments	$ 67,627	$ 67,627
Redemption Notice Period	1 day	1 day
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	$ 67,627
Fair value of plan assets at end of year	74,511	$ 67,627
Qualified Plan | Qualified Pension Plan | Equity securities: Fidelity Spartan 500 index fund
Fair value hierarchy of Plan's assets
Total investments	$ 58,815	$ 58,815
Redemption Notice Period	1 day	1 day
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	$ 58,815
Fair value of plan assets at end of year	71,632	$ 58,815
Qualified Plan | Qualified Pension Plan | Level 1
Fair value hierarchy of Plan's assets
Total investments	62,990	62,990
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	62,990
Fair value of plan assets at end of year	111,529	62,990
Qualified Plan | Qualified Pension Plan | Level 1 | Collective short-term investment fund
Fair value hierarchy of Plan's assets
Total investments	4,175	4,175
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	4,175
Fair value of plan assets at end of year	39,897	4,175
Qualified Plan | Qualified Pension Plan | Level 1 | Equity index funds
Fair value hierarchy of Plan's assets
Total investments	58,815	58,815
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	58,815
Fair value of plan assets at end of year	71,632	58,815
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	67,627	67,627
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	67,627
Fair value of plan assets at end of year	74,511	67,627
Qualified Plan | Qualified Pension Plan | Level 2 | Collective short-term investment 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 | Level 2 | Equity index funds
Fair value hierarchy of Plan's assets
Total investments	67,627	67,627
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	67,627
Fair value of plan assets at end of year	74,511	67,627
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	71,226	67,707
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	71,226	67,707
Interest income	3,660	3,519
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	74,886	71,226
Qualified Plan | Qualified Pension Plan | Level 3 | Collective short-term investment 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 | Level 3 | Equity index funds
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 | Group deposit administration annuity contract
Fair value hierarchy of Plan's assets
Total investments	71,226	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	71,226
Fair value of plan assets at end of year	$ 74,886	$ 71,226