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Marketable Securities (Details) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
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 $ 257,570 $ 261,208
Amortized Cost 257,614 261,340
Gross Unrealized Gains 1 0
Gross Unrealized Losses (45) (132)
Cash, fair value 40,770 68,168
Cash, amortized cost 40,770 68,168
Cash, cash equivalents and marketable securities    
Fair Value 298,340 329,376
Amortized Cost 298,384 329,508
Gross Unrealized Gains 1 0
Gross Unrealized Losses (45) (132)
Money market funds    
Cash equivalents and marketable securities    
Fair Value 11,078 10,915
Amortized Cost 11,078 10,915
Gross Unrealized Gains 0 0
Gross Unrealized Losses $ 0 $ 0
Weighted Rate of Return (as a percent) 1.75% 1.16%
US Treasury and Government Short-term Debt Securities [Member]    
Cash equivalents and marketable securities    
Fair Value $ 68,898 $ 55,220
Amortized Cost 68,897 55,221
Gross Unrealized Gains 1 0
Gross Unrealized Losses $ 0 $ (1)
Weighted Rate of Return (as a percent) 1.74% 1.12%
Corporate notes, bonds and commercial paper    
Cash equivalents and marketable securities    
Fair Value $ 177,594 $ 195,073
Amortized Cost 177,639 195,204
Gross Unrealized Gains 0 0
Gross Unrealized Losses $ (45) $ (131)
Weighted Rate of Return (as a percent) 1.90% 1.39%