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Cash, Cash Equivalents and Investments (Summary of Cash, Cash Equivalents and Investments) (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 82,866 $ 102,390
Gross unrealized gains 12 3
Gross unrealized losses (6) (39)
Fair value 82,872 102,354
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 38,206 18,844
Gross unrealized gains
Gross unrealized losses (1)
Fair value 38,205 18,844
Cash and money market funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 29,329 18,844
Gross unrealized gains
Gross unrealized losses
Fair value 29,329 18,844
Commercial paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 8,877  
Gross unrealized gains
Gross unrealized losses (1)
Fair value 8,876  
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 44,660 83,546
Gross unrealized gains 12 3
Gross unrealized losses (5) (39)
Fair value 44,667 83,510
Commercial paper, not included with cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 31,309 53,493
Gross unrealized gains 1
Gross unrealized losses (4) (24)
Fair value 31,306 53,469
Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 6,970 10,223
Gross unrealized gains 10 3
Gross unrealized losses (1) (12)
Fair value 6,979 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 6,381 19,830
Gross unrealized gains 1
Gross unrealized losses (3)
Fair value 6,382 19,827
Cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 4,500 4,500
Gross unrealized gains
Gross unrealized losses
Fair value $ 4,500 $ 4,500