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Cash, cash equivalents and short-term investments - Narrative (Details)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
security
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
security
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]            
Debt securities in an unrealized position for less than 12 months (in securities) | security 5   5   10 21
Aggregate fair value of securities in an unrealized loss position for less than 12 months $ 10,400,000   $ 10,400,000   $ 22,900,000 $ 46,600,000
Number of investments in a continuous unrealized loss position (in securities) | security 5   5      
Securities held $ 53,637,000   $ 53,637,000   67,230,000 114,448,000
Debt securities            
Schedule of Available-for-sale Securities [Line Items]            
Securities held 7,900,000   7,900,000   12,000,000 11,500,000
Short-term investments            
Schedule of Available-for-sale Securities [Line Items]            
Securities held 29,036,000   29,036,000   $ 51,527,000 $ 70,518,000
Realized gains on investments 0 $ 0 0 $ 0    
Realized losses on investments $ 0 $ 0 $ 0 $ 0