EMPLOYEE BENEFIT PLANS - Level 3 Financial Instrument (Details) - Qualified Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Level 3 | Group deposit administration annuity contract | Contract Value | Minimum
Employee benefit plans
Contract rate (as a percent)	5.10%
Level 3 | Group deposit administration annuity contract | Contract Value | Maximum
Employee benefit plans
Contract rate (as a percent)	5.19%
Qualified Pension Plan
Employee benefit plans
Fair Value	$ 260,926	$ 201,843	$ 196,042
Qualified Pension Plan | Level 3
Employee benefit plans
Fair Value	74,886	71,226	$ 67,707
Group deposit administration annuity contract | Qualified Pension Plan
Employee benefit plans
Fair Value	74,886	71,226
Group deposit administration annuity contract | Qualified Pension Plan | Level 3
Employee benefit plans
Fair Value	$ 74,886	$ 71,226