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Short-term and long-term investments (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Investment Holdings [Line Items]    
Debt securities $ 2,235 $ 31,630
Equity Securities, FV-NI 36,124  
Available for sale   38,895
Total 38,359 70,525
Held to maturity, short-term investments 2,235 31,630
Available for sale, short-term investments   1,200
Total, short-term investments 2,935 32,830
Held to maturity, long-term investments 0 0
Available for sale, long-term investments   37,695
Total, long-term investments 35,424 37,695
Certificates of deposit and other time deposits    
Investment Holdings [Line Items]    
Debt securities 2,235 31,630
Equity Securities, FV-NI 0  
Available for sale, equity securities   0
Total 2,235 31,630
Investments in mutual funds and common stock    
Investment Holdings [Line Items]    
Debt securities 0 0
Equity Securities, FV-NI 36,124  
Available for sale, equity securities   38,895
Total 36,124 $ 38,895
Short-term Investments    
Investment Holdings [Line Items]    
Equity Securities, FV-NI 700  
Other Long-term Investments    
Investment Holdings [Line Items]    
Equity Securities, FV-NI $ 35,424