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 equivalents - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Adjusted Cost $ 85,363 $ 73,633
Gross Unrealized Gains/Losses (33) (25)
Fair Value 85,330 73,608
Municipal bonds and notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Adjusted Cost 12,149 13,320
Gross Unrealized Gains/Losses (7) (1)
Fair Value 12,142 13,319
Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Adjusted Cost 51,862 49,432
Gross Unrealized Gains/Losses (26) (24)
Fair Value 51,836 49,408
Financial institution instruments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Adjusted Cost 21,352 10,881
Gross Unrealized Gains/Losses 0 0
Fair Value $ 21,352 $ 10,881