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Bonds Payable - Summary of Bonds Payable (Parenthetical) (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Jul. 31, 2015
Repayable at maturity in January 2021 [member]      
Disclosure of detailed information about borrowings [line items]      
Interest rate 1.30% 1.30%  
Repayable at maturity in January 2023 [member]      
Disclosure of detailed information about borrowings [line items]      
Interest rate 1.50% 1.50%  
Repayable at maturity in January 2022 [member]      
Disclosure of detailed information about borrowings [line items]      
Interest rate 1.25% 1.25%  
Repayable at maturity in January 2024 [member]      
Disclosure of detailed information about borrowings [line items]      
Interest rate 1.45% 1.45%  
US$200,000 thousand [member]      
Disclosure of detailed information about borrowings [line items]      
Notional Amount $ 200,000   $ 200,000