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Cash, Cash Equivalents and Investments (Summary of Cash, Cash Equivalents and Investments) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 115,771 $ 62,904
Gross unrealized gains
Gross unrealized losses (24) (32)
Fair value 115,747 62,872
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 38,065 16,507
Gross unrealized gains
Gross unrealized losses (2)
Fair value 38,063 16,507
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 77,706 46,397
Gross unrealized gains
Gross unrealized losses (22) (32)
Fair value 77,684 46,365
Cash and money market funds [Member] | Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 27,386 14,858
Gross unrealized gains
Gross unrealized losses
Fair value 27,386 14,858
Commercial paper [Member] | Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 10,679 1,649
Gross unrealized gains
Gross unrealized losses (2)
Fair value 10,677 1,649
Commercial paper [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 51,647 20,408
Gross unrealized gains
Gross unrealized losses (14) (14)
Fair value 51,633 20,394
Corporate debt securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 9,070 9,043
Gross unrealized gains
Gross unrealized losses (2) (9)
Fair value 9,068 9,034
U.S. government and agency securities [Member] | Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 16,989 16,946
Gross unrealized gains
Gross unrealized losses (6) (9)
Fair value 16,983 16,937
Cash [Member] | Long-term restricted 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