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Cash, Cash Equivalents and Investments (Summary of Cash, Cash Equivalents and Investments) (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 49,193 $ 102,390
Gross unrealized gains 11 3
Gross unrealized losses (39)
Fair value 49,204 102,354
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 33,084 18,844
Gross unrealized gains
Gross unrealized losses
Fair value 33,084 18,844
Cash and money market funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 18,505 18,844
Gross unrealized gains
Gross unrealized losses
Fair value 18,505 18,844
Commercial paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 14,579  
Gross unrealized gains  
Gross unrealized losses  
Fair value 14,579  
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 16,109 83,546
Gross unrealized gains 11 3
Gross unrealized losses (39)
Fair value 16,120 83,510
Commercial paper, not included with cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 12,117 53,493
Gross unrealized gains 1
Gross unrealized losses (24)
Fair value 12,118 53,469
Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 3,992 10,223
Gross unrealized gains 10 3
Gross unrealized losses (12)
Fair value 4,002 10,214
U.S. government and agency securities, not included with cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost   19,830
Gross unrealized gains  
Gross unrealized losses   (3)
Fair value   19,827
Certified deposits [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 4,000 4,500
Gross unrealized gains
Gross unrealized losses
Fair value $ 4,000 $ 4,500