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Share-Based Compensation - Additional Information (Detail) - Class_of_Stock
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Description of share based compensation The Company currently has two LTI modes stock options and virtual shares.  
Option grant date Last sixty trading days  
Number of trading days 10 trading days  
Restricted virtual shares [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Percentage of third anniversary grant date   33.00%
Percentage of fourth anniversary grant date   33.00%
Percentage of fifth anniversary grant date   34.00%
Top of range [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Percentage adjusted to offset speculation 30.00%  
Top of range [member] | Option Granted For Others [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Option exercise period P7Y  
Phantom Share Plans [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Number of trading days 30 trading days  
Number of classes 2  
Phantom Share Plans [member] | Restricted virtual shares [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Percentage of shares issued 50.00%  
Phantom Share Plans [member] | Virtual performance shares [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Percentage of shares issued 50.00%  
Grant Date Four [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Vesting, Conditions 33% after 3 years  
Grant Date Five [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Vesting, Conditions 33% after 4 years  
Grant Date Six [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Vesting, Conditions 34% after 5 years