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Summary of Significant Accounting Policies (Cash and Equivalents) (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents $ 144,353 $ 93,108 $ 32,335 $ 119,471
Cash [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 17,481 29,569    
Money market funds [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 95,896 14,950    
Government-backed securities [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 19,000 20,000    
Asset-backed Securities [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 0 8,185    
Corporate debt securities [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents $ 11,976 $ 20,404