EMPLOYEE BENEFIT PLANS - Plan Assets of the Defined Benefit Pension Plan By Category (Details) - Qualified Plan - Qualified Pension Plan - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
EMPLOYEE BENEFIT PLANS
Total investments	$ 260,926	$ 201,843	$ 196,042
Employer contribution receivable	0	0
Total	260,926	201,843
Cash and cash equivalents
EMPLOYEE BENEFIT PLANS
Total investments	39,897	4,175
Equity securities: Collective Russell 3000 index fund
EMPLOYEE BENEFIT PLANS
Total investments	74,511	67,627
Equity securities: Fidelity Spartan 500 index fund
EMPLOYEE BENEFIT PLANS
Total investments	71,632	58,815
Fixed income
EMPLOYEE BENEFIT PLANS
Total investments	$ 74,886	$ 71,226