FAIR VALUE MEASUREMENTS - Available-for-Sale Investments Included in Cash Equivalents (Details) - Cash and Cash Equivalents - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	$ 85,993	$ 85,363
Gross Unrealized Losses	(16)	(33)
Fair Value	85,977	85,330
Municipal bonds and notes
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	4,188	12,149
Gross Unrealized Losses	0	(7)
Fair Value	4,188	12,142
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	48,535	51,862
Gross Unrealized Losses	(16)	(26)
Fair Value	48,519	51,836
Financial institution instruments
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	33,270	21,352
Gross Unrealized Losses	0	0
Fair Value	$ 33,270	$ 21,352