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Cash and Cash Equivalents and Investments (Summary of Cash, Cash Equivalents and Investments) (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 142,561 $ 49,094
Gross unrealized gains 105 6
Gross unrealized losses (75) (1)
Fair value 142,591 49,099
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 68,857 29,627
Gross unrealized gains 6
Gross unrealized losses (1)
Fair value 68,862 29,627
Cash and money market funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 32,529 18,644
Gross unrealized gains
Gross unrealized losses
Fair value 32,529 18,644
Commercial paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 36,328 10,983
Gross unrealized gains 6
Gross unrealized losses (1)
Fair value 36,333 10,983
U.S. government & agency securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 15,009  
Gross unrealized gains 86  
Gross unrealized losses  
Fair value 15,095  
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 73,704 19,467
Gross unrealized gains 99 6
Gross unrealized losses (74) (1)
Fair value 73,729 19,472
Commercial paper, not included with cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 45,697 16,971
Gross unrealized gains 13 1
Gross unrealized losses (1)
Fair value 45,710 16,971
Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 12,998 2,496
Gross unrealized gains 5
Gross unrealized losses (74)
Fair value 12,924 2,501
Long-term restricted cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 4,000 4,000
Gross unrealized gains
Gross unrealized losses
Fair value $ 4,000 $ 4,000