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Summary of Stock Option Activity (Parenthetical) (Detail) - shares
 6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Outstanding amount of stock appreciation right units [2] 714,645 [1] 729,017
Exercisable amount of stock appreciation right units [2],[3] 569,977  
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Outstanding amount of stock appreciation right units 651,296  
Exercisable amount of stock appreciation right units 515,045  
Stock Appreciation Rights (SARs) | Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Maximum percentage of stock appreciation right units 75.00%  
[1] Due to the ceiling imposed on the SAR grants, the outstanding amount equals a maximum of 651,296 shares of the Company's common stock issuable upon exercise.
[2] The SAR units are convertible for a maximum number of shares of the Company's common stock equal to 75% of the SAR units subject to the grant.
[3] Due to the ceiling imposed on the SAR grants, the exercisable amount equals a maximum of 515,045 shares of the Company's common stock issuable upon exercise.