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Share-Based Compensation - Additional Information (Detail) - shares
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
May. 21, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Options granted     2,919,750 2,289,600  
Issuance of shares under stock purchase plan     0 0  
Stock Option Plan 2015          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Shares authorized for grant         3,000,000
Directors' Restricted Stock Plan 2014          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Shares granted to independent directors 33,868 30,702