EMPLOYEE BENEFIT PLANS - Plan Assets of the Defined Benefit Pension Plan By Category (Details) - Qualified Plan - Qualified Pension Plan - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
EMPLOYEE BENEFIT PLANS
Total investments	$ 253,955	$ 260,926	$ 201,843
Employer contribution receivable	0	0
Total	253,955	260,926
Cash and cash equivalents
EMPLOYEE BENEFIT PLANS
Total investments	1,225	39,897
Collective Russell 3000 equity index fund
EMPLOYEE BENEFIT PLANS
Total investments	70,599	74,511
Fidelity Spartan 500 index fund
EMPLOYEE BENEFIT PLANS
Total investments	46,300	71,632
Northern Trust ACWI ex-US Fund
EMPLOYEE BENEFIT PLANS
Total investments	41,924	0
Group deposit administration annuity contract
EMPLOYEE BENEFIT PLANS
Total investments	78,707	74,886
BlackRock Long Government Credit Bond Index Fund
EMPLOYEE BENEFIT PLANS
Total investments	$ 15,200	$ 0