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Short-term and long-term investments - Additional Information (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2018
Debt and Equity Securities, FV-NI [Line Items]      
Pre tax reclassification of net investment realized gain into net income   $ 360  
Reclassification of net investment realized gain into net income, net of tax   $ 220  
Investments in mutual funds and common stock      
Debt and Equity Securities, FV-NI [Line Items]      
Equity Securities, FV-NI, Gain (Loss) $ 1,208    
Equity Securities, FV-NI, Realized Gain 4,490    
Equity Securities, FV-NI, Unrealized Gain $ 5,698    
Investment securities      
Debt and Equity Securities, FV-NI [Line Items]      
Cumulative Effect of New Accounting Principle in Period of Adoption     $ (5,662)