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Investments in debt and equity securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Investment Holdings [Line Items]    
Held to maturity $ 98,113 $ 2,235
Held to maturity, short-term investments 98,113 2,235
Held to maturity, long-term investments 0 0
Equity Securities, FV-NI 37,641 36,124
Total 135,754 38,359
Total, short-term investments 100,713 2,935
Total, long-term investments 35,041 35,424
Certificates of deposit and other time deposits    
Investment Holdings [Line Items]    
Held to maturity 98,113 2,235
Equity Securities, FV-NI 0 0
Total 98,113 2,235
Investments in mutual funds and common stock    
Investment Holdings [Line Items]    
Held to maturity 0 0
Equity Securities, FV-NI 37,641 36,124
Total 37,641 36,124
Short-term Investments    
Investment Holdings [Line Items]    
Equity Securities, FV-NI 2,600 700
Long-term Investments    
Investment Holdings [Line Items]    
Equity Securities, FV-NI $ 35,041 $ 35,424