Annual report pursuant to Section 13 and 15(d)

FAIR VALUE MEASUREMENTS - Available-for-sale Investments Included in Cash Equivalents (Details)

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FAIR VALUE MEASUREMENTS - Available-for-sale Investments Included in Cash Equivalents (Details) - Cash and Cash Equivalents - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Adjusted Cost $ 82,334 $ 85,363
Gross Unrealized Losses (19) (33)
Fair Value 82,315 85,330
Level 1 | Money market funds    
Debt Securities, Available-for-sale [Line Items]    
Adjusted Cost 11,659 3,281
Gross Unrealized Losses 0 0
Fair Value 11,659 3,281
Level 2 | Time deposits    
Debt Securities, Available-for-sale [Line Items]    
Adjusted Cost 13,544 18,071
Gross Unrealized Losses 0 0
Fair Value 13,544 18,071
Level 2 | Municipal debt securities    
Debt Securities, Available-for-sale [Line Items]    
Adjusted Cost 347 12,149
Gross Unrealized Losses 0 (7)
Fair Value 347 12,142
Level 2 | Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Adjusted Cost 56,784 51,862
Gross Unrealized Losses (19) (26)
Fair Value $ 56,765 $ 51,836