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Marketable Securities (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 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 $ 232,142 $ 261,208
Amortized Cost 232,326 261,340
Gross Unrealized Gains 0 0
Gross Unrealized Losses (184) (132)
Cash, fair value 59,078 68,168
Cash, amortized cost 59,078 68,168
Cash, cash equivalents and marketable securities    
Fair Value 291,220 329,376
Amortized Cost 291,404 329,508
Gross Unrealized Gains 0 0
Gross Unrealized Losses (184) (132)
Money market funds    
Cash equivalents and marketable securities    
Fair Value 11,230 10,915
Amortized Cost 11,230 10,915
Gross Unrealized Gains 0 0
Gross Unrealized Losses $ 0 $ 0
Weighted Rate of Return (as a percent) 1.51% 1.16%
US Treasury and Government Short-term Debt Securities [Member]    
Cash equivalents and marketable securities    
Fair Value $ 22,913 $ 55,220
Amortized Cost 22,916 55,221
Gross Unrealized Gains 0 0
Gross Unrealized Losses $ (3) $ (1)
Weighted Rate of Return (as a percent) 1.64% 1.12%
Corporate notes, bonds and commercial paper    
Cash equivalents and marketable securities    
Fair Value $ 197,999 $ 195,073
Amortized Cost 198,180 195,204
Gross Unrealized Gains 0 0
Gross Unrealized Losses $ (181) $ (131)
Weighted Rate of Return (as a percent) 1.65% 1.39%