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Cash, cash equivalents and short-term investments - Narrative (Details)
$ in Thousands
Jun. 30, 2016
USD ($)
security
Dec. 31, 2015
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]    
Debt securities in an unrealized position for less than 12 months (in securities) | security 6 43
Number of investments in a continuous unrealized loss position for 12 months or longer (in securities) | security 5  
Number of investments in a continuous unrealized loss position (in securities) | security 11  
Aggregate fair value of securities in an unrealized loss position for less than 12 months $ 10,800 $ 81,400
Fair value of securities in an unrealized loss position for longer than 12 months 6,600  
Securities held 147,644 126,832
Debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Securities held $ 5,300 $ 11,700