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Bonds Payable - Summary of Bonds Payable (Parenthetical) (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Jul. 31, 2015
Jul. 31, 2014
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%      
Repayable at maturity in January 2024 [member]        
Disclosure of detailed information about borrowings [line items]        
Interest rate 1.45%      
US$400,000 thousand [member] | Cost [member]        
Disclosure of detailed information about borrowings [line items]        
Notional Amount $ 400,000,000      
US$200,000 thousand [member]        
Disclosure of detailed information about borrowings [line items]        
Notional Amount     $ 200,000,000  
US$200,000 thousand [member] | Cost [member]        
Disclosure of detailed information about borrowings [line items]        
Notional Amount 200,000,000      
Secured Overseas Bonds One [member]        
Disclosure of detailed information about borrowings [line items]        
Interest rate   2.125%   2.125%
Notional Amount       $ 300,000,000
Secured Overseas Bonds One [member] | Cost [member]        
Disclosure of detailed information about borrowings [line items]        
Notional Amount $ 300,000,000      
Bonds repayment date July 2017