Quarterly report pursuant to Section 13 or 15(d)

Cash, Cash Equivalents, and Short-Term Investments (Details)

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Cash, Cash Equivalents, and Short-Term Investments (Details)
$ in Thousands
Sep. 30, 2017
USD ($)
investment
Dec. 31, 2016
USD ($)
Sep. 30, 2016
USD ($)
Dec. 31, 2015
USD ($)
Cash and Cash Equivalents [Line Items]        
Cash $ 132,405 $ 213,551 $ 81,073 $ 82,469
Schedule of Available-for-sale Securities [Line Items]        
Short-term investments 0 34,019    
Total cash, cash equivalents and short-term investments $ 132,405 247,570    
Number of investments have been in a continuous loss position greater than one year | investment 0      
U.S. investment grade bonds        
Schedule of Available-for-sale Securities [Line Items]        
Short-term investments $ 0 24,477    
Foreign investment grade bonds        
Schedule of Available-for-sale Securities [Line Items]        
Short-term investments 0 9,542    
Cash        
Cash and Cash Equivalents [Line Items]        
Cash $ 132,405 $ 213,551