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Summary of Significant Accounting Policies (Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Assets    
Total assets measured at fair value $ 120,247 $ 67,372
Liabilities    
Total liabilities measured at fair value 28 183
Financing Derivative [Member]    
Liabilities    
Total liabilities measured at fair value 28 183
Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 38,063 16,507
Cash and cash equivalents [Member] | Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 27,386 14,858
Cash and cash equivalents [Member] | Commercial paper [Member]    
Assets    
Total cash and cash equivalents 10,677 1,649
Investments [Member]    
Assets    
Total investments 77,684 46,365
Investments [Member] | Commercial paper [Member]    
Assets    
Total investments 51,633 20,394
Investments [Member] | Corporate debt securities [Member]    
Assets    
Total investments 9,068 9,034
Investments [Member] | U.S. government and agency securities [Member]    
Assets    
Total investments 16,983 16,937
Long-term restricted cash [Member] | Cash [Member]    
Assets    
Long-term restricted cash 4,500 4,500
Level 1 [Member]    
Assets    
Total assets measured at fair value 31,886 19,358
Level 1 [Member] | Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 27,386 14,858
Level 1 [Member] | Cash and cash equivalents [Member] | Cash and money market funds [Member]    
Assets    
Total cash and cash equivalents 27,386 14,858
Level 1 [Member] | Long-term restricted cash [Member] | Cash [Member]    
Assets    
Long-term restricted cash 4,500 4,500
Level 2 [Member]    
Assets    
Total assets measured at fair value 88,361 48,014
Level 2 [Member] | Cash and cash equivalents [Member]    
Assets    
Total cash and cash equivalents 10,677 1,649
Level 2 [Member] | Cash and cash equivalents [Member] | Commercial paper [Member]    
Assets    
Total cash and cash equivalents 10,677 1,649
Level 2 [Member] | Investments [Member]    
Assets    
Total investments 77,684 46,365
Level 2 [Member] | Investments [Member] | Commercial paper [Member]    
Assets    
Total investments 51,633 20,394
Level 2 [Member] | Investments [Member] | Corporate debt securities [Member]    
Assets    
Total investments 9,068 9,034
Level 2 [Member] | Investments [Member] | U.S. government and agency securities [Member]    
Assets    
Total investments 16,983 16,937
Level 3 [Member]    
Liabilities    
Total liabilities measured at fair value 28 183
Level 3 [Member] | Financing Derivative [Member]    
Liabilities    
Total liabilities measured at fair value $ 28 $ 183