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Fair value measurements (Details) - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities $ 93,315 $ 194,386
Cash and cash equivalents 126,600 228,600
Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 4,954 51,059
Marketable securities 93,315 194,386
Marketable securities with contractual maturity of one year or less 93,315 122,199
Marketable securities with contractual maturity of one to two years 0 72,187
Fair Value, Measurements, Recurring [Member] | US Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 13,334 14,451
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 0 2,197
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 77,621 165,825
Fair Value, Measurements, Recurring [Member] | Municipal Notes [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 2,360 11,913
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 4,954 51,059
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 4,954 51,059
Marketable securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | US Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Municipal Notes [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 4,954 51,059
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Marketable securities 93,315 194,386
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | US Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 13,334 14,451
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 0 2,197
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 77,621 165,825
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipal Notes [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 2,360 11,913
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 0 $ 0