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Share-Based Compensation - Schedule of Outstanding and Exercisable Options Granted (Detail)
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]  
Outstanding 154,726
Exercisable 154,726
35.01 - 40.00 [Member]  
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]  
Lower exercise price | $ / shares $ 35.01
Upper exercise price | $ / shares $ 40.00
Outstanding 149,188
Remaining Contractual Life (Years) 4 years 4 months 24 days
Exercisable 149,188
45.01 - 50.00 [Member]  
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]  
Lower exercise price | $ / shares $ 45.01
Upper exercise price | $ / shares $ 50.00
Outstanding 5,538
Remaining Contractual Life (Years) 6 years
Exercisable 5,538