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Restricted Stock, Performance Share and Cash Awards - Schedule of Restricted Stock Awards Vesting Periods (Detail) - shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Vesting Period [Line Items]      
Shares granted in the period 476,350 479,167 394,975
Vesting Period One Year [Member]      
Vesting Period [Line Items]      
Shares granted in the period 21,600 27,417 22,175
Vesting period, years 1 year    
Vesting Period Four Years [Member]      
Vesting Period [Line Items]      
Shares granted in the period     9,200
Vesting period, years 4 years    
Vesting Period Five Years [Member]      
Vesting Period [Line Items]      
Shares granted in the period 454,750 451,750 363,600
Vesting period, years 5 years