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Cash, Cash Equivalents and Investments (Summary of Cash, Cash Equivalents and Investments) (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 66,827 $ 102,390
Gross unrealized gains 21 3
Gross unrealized losses (4) (39)
Fair value 66,844 102,354
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 30,360 18,844
Gross unrealized gains
Gross unrealized losses (2)
Fair value 30,358 18,844
Cash and money market funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 17,124 18,844
Gross unrealized gains
Gross unrealized losses
Fair value 17,124 18,844
Commercial paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 10,388  
Gross unrealized gains  
Gross unrealized losses (2)  
Fair value 10,386  
U.S. Treasury securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 2,848  
Gross unrealized gains  
Gross unrealized losses  
Fair value 2,848  
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 36,467 83,546
Gross unrealized gains 21 3
Gross unrealized losses (2) (39)
Fair value 36,486 83,510
Commercial paper, not included with cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 29,590 53,493
Gross unrealized gains 4
Gross unrealized losses (2) (24)
Fair value 29,592 53,469
Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 5,481 10,223
Gross unrealized gains 16 3
Gross unrealized losses (12)
Fair value 5,497 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 1,396 19,830
Gross unrealized gains 1
Gross unrealized losses (3)
Fair value 1,397 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