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Marketable Securities (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2016
Dec. 31, 2015
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 $ 179,285 $ 252,737
Amortized Cost 179,313 252,925
Gross Unrealized Gains 2 0
Gross Unrealized Losses (30) (188)
Cash, fair value 46,287 34,969
Cash, amortized cost 46,287 34,969
Cash, cash equivalents and marketable securities    
Fair Value 225,572 287,706
Amortized Cost 225,600 287,894
Gross Unrealized Gains 2 0
Gross Unrealized Losses (30) (188)
Money market funds    
Cash equivalents and marketable securities    
Fair Value 90,342 77,804
Amortized Cost 90,342 77,804
Gross Unrealized Gains 0 0
Gross Unrealized Losses $ 0 $ 0
Weighted Rate of Return (as a percent) 0.20% 0.12%
US Treasury and Government Short-term Debt Securities [Member]    
Cash equivalents and marketable securities    
Fair Value $ 14,057 $ 14,110
Amortized Cost 14,061 14,142
Gross Unrealized Gains 0 0
Gross Unrealized Losses $ (4) $ (32)
Weighted Rate of Return (as a percent) 0.48% 0.48%
Corporate notes, bonds and commercial paper    
Cash equivalents and marketable securities    
Fair Value $ 74,886 $ 160,823
Amortized Cost 74,910 160,979
Gross Unrealized Gains 2 0
Gross Unrealized Losses $ (26) $ (156)
Weighted Rate of Return (as a percent) 0.56% 0.45%