FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	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	$ 5,021	$ 6,088
Liabilities fair value disclosure	1,217
Recurring | Interest rate swap agreement
Assets measured at fair value on a recurring basis
Assets fair value disclosure		1,147
Liabilities fair value disclosure	1,217
Recurring | Fixed-income securities
Assets measured at fair value on a recurring basis
Assets fair value disclosure	4,203	4,157
Recurring | Money market funds
Assets measured at fair value on a recurring basis
Assets fair value disclosure	818	784
Recurring | Quoted Prices in Active Markets (Level 1)
Assets measured at fair value on a recurring basis
Assets fair value disclosure	2,797	2,345
Recurring | Quoted Prices in Active Markets (Level 1) | Fixed-income securities
Assets measured at fair value on a recurring basis
Assets fair value disclosure	1,979	1,561
Recurring | Quoted Prices in Active Markets (Level 1) | Money market funds
Assets measured at fair value on a recurring basis
Assets fair value disclosure	818	784
Recurring | Other Observable Inputs (Level 2)
Assets measured at fair value on a recurring basis
Assets fair value disclosure	2,224	3,743
Liabilities fair value disclosure	1,217
Recurring | Other Observable Inputs (Level 2) | Interest rate swap agreement
Assets measured at fair value on a recurring basis
Assets fair value disclosure		1,147
Liabilities fair value disclosure	1,217
Recurring | Other Observable Inputs (Level 2) | Fixed-income securities
Assets measured at fair value on a recurring basis
Assets fair value disclosure	$ 2,224	$ 2,596