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Marketable Securities (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Cash equivalents and marketable securities      
Maximum maturity period of available-for-sale securities (in years) 3 years 3 years  
Maximum remaining maturity period of available-for-sale securities (in years) 1 year 1 year  
Fair Value $ 133,697   $ 121,151
Amortized Cost 133,719   121,171
Gross Unrealized Gains 1   1
Gross Unrealized Losses (23)   (21)
Cash, fair value 53,928   51,031
Cash, amortized cost 53,928   51,031
Cash, cash equivalents and marketable securities      
Fair Value 187,625   172,182
Amortized Cost 187,647   172,202
Gross Unrealized Gains 1   1
Gross Unrealized Losses (23)   (21)
Money market funds      
Cash equivalents and marketable securities      
Fair Value 11,075   10,681
Amortized Cost 11,075   10,681
Gross Unrealized Gains 0   0
Gross Unrealized Losses $ 0   $ 0
Weighted Rate of Return (as a percent) 0.60%   0.41%
US Treasury and Government Short-term Debt Securities [Member]      
Cash equivalents and marketable securities      
Fair Value $ 35,975   $ 48,292
Amortized Cost 35,975   48,291
Gross Unrealized Gains 1   1
Gross Unrealized Losses $ (1)   $ 0
Weighted Rate of Return (as a percent) 0.72%   0.39%
Corporate notes, bonds and commercial paper      
Cash equivalents and marketable securities      
Fair Value $ 86,647   $ 62,178
Amortized Cost 86,669   62,199
Gross Unrealized Gains 0   0
Gross Unrealized Losses $ (22)   $ (21)
Weighted Rate of Return (as a percent) 0.80%   0.66%