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Investments and Fair Value Measurements - Recurring basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities $ 78,287 $ 53,723
Recurring [Member]    
Assets, Fair Value Disclosure [Abstract]    
Total 31,233 29,775
Recurring [Member] | Certificates of Deposit [Member]    
Assets, Fair Value Disclosure [Abstract]    
Cash and cash equivalents, fair value disclosure 7,631 21,184
Recurring [Member] | Corporate Bonds [Member]    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities 23,602 8,432
Recurring [Member] | Corporate Equity Securities [Member]    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities   159
Recurring [Member] | Quoted Prices in Active Markets of Identical Assets (Level 1) [Member]    
Assets, Fair Value Disclosure [Abstract]    
Total   159
Recurring [Member] | Quoted Prices in Active Markets of Identical Assets (Level 1) [Member] | Corporate Equity Securities [Member]    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities   159
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]    
Assets, Fair Value Disclosure [Abstract]    
Total 31,233 29,616
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]    
Assets, Fair Value Disclosure [Abstract]    
Cash and cash equivalents, fair value disclosure 7,631 21,184
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]    
Assets, Fair Value Disclosure [Abstract]    
Available-for-sale Securities $ 23,602 $ 8,432