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Bank revolving credit notes - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Jun. 30, 2019
Revolving credit notes $ 59,629 $ 41,485
Revolving Credit Facility [Member]    
Credit agreement borrowing capacity   $ 90,000
Interest rate 4.69% 4.75%
Revolving commitments fee percentage 0.20%  
Revolving credit notes $ 59,629 $ 41,485