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Summary of Stock Option Activity (Parenthetical) (Detail) - shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Outstanding amount of stock appreciation right units [2] 729,017 [1] 1,455,908
Exercisable amount of stock appreciation right units [2],[3] 513,464  
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Outstanding amount of stock appreciation right units 662,075  
Exercisable amount of stock appreciation right units 465,223  
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 662,075 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 465,223 shares of the Company's common stock issuable upon exercise.