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Fair value measurements (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities with contractual maturity of one year or less $ 122,199 $ 58,764
Marketable securities with contractual maturity of one to two years 72,187 43,563
Marketable securities 194,386 102,327
Cash and cash equivalents 228,600 237,000
Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 51,059 82,968
Marketable securities 194,386 102,327
Fair Value, Measurements, Recurring [Member] | Municipal Notes [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 11,913 0
Fair Value, Measurements, Recurring [Member] | US Treasury Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 0 1,994
Fair Value, Measurements, Recurring [Member] | US Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 14,451 7,020
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 2,197 2,497
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 165,825 90,816
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 51,059 80,968
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 2,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 51,059 80,968
Marketable securities 0 1,994
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] | US Treasury Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 0 1,994
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] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 51,059 80,968
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]    
Cash equivalents 0 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 2,000
Marketable securities 194,386 100,333
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 11,913 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | US Treasury 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 2 [Member] | US Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 14,451 7,020
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 2,197 2,497
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 165,825 90,816
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
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]    
Cash equivalents $ 0 $ 2,000