FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2014
Interest rate swap agreement
Assets measured at fair value on a recurring basis
Initial notional amount			$ 250,000
Recurring
Assets measured at fair value on a recurring basis
Assets fair value disclosure	$ 7,178	$ 5,802
Liabilities fair value disclosure		638
Recurring | Interest rate swap agreement
Assets measured at fair value on a recurring basis
Assets fair value disclosure	1,385
Liabilities fair value disclosure		638
Recurring | Fixed-income securities
Assets measured at fair value on a recurring basis
Assets fair value disclosure	4,127	4,045
Recurring | Money market funds
Assets measured at fair value on a recurring basis
Assets fair value disclosure	1,666	1,757
Recurring | Quoted Prices in Active Markets (Level 1)
Assets measured at fair value on a recurring basis
Assets fair value disclosure	3,051	3,153
Recurring | Quoted Prices in Active Markets (Level 1) | Fixed-income securities
Assets measured at fair value on a recurring basis
Assets fair value disclosure	1,385	1,396
Recurring | Quoted Prices in Active Markets (Level 1) | Money market funds
Assets measured at fair value on a recurring basis
Assets fair value disclosure	1,666	1,757
Recurring | Other Observable Inputs (Level 2)
Assets measured at fair value on a recurring basis
Assets fair value disclosure	4,127	2,649
Liabilities fair value disclosure		638
Recurring | Other Observable Inputs (Level 2) | Interest rate swap agreement
Assets measured at fair value on a recurring basis
Assets fair value disclosure	1,385
Liabilities fair value disclosure		638
Recurring | Other Observable Inputs (Level 2) | Fixed-income securities
Assets measured at fair value on a recurring basis
Assets fair value disclosure	$ 2,742	$ 2,649