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Stock-Based Compensation - Analysis of Unvested Restricted Stock (Detail) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Beginning Balance (in shares) 491,768 646,682 819,790
Grants (in shares) 169,870 186,961 193,788
Additional performance shares 106,084 (35,073) (58,056)
Restriction lapses (in shares) (153,360) (306,802) (287,390)
Forfeitures (in shares) (15,000) 0 (21,450)
Ending Balance (in shares) 599,362 491,768 646,682
Executive Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Beginning Balance (in shares) 57,450 187,665 263,430
Grants (in shares) 0 0 0
Additional performance shares
Restriction lapses (in shares) (14,700) (130,215) (61,815)
Forfeitures (in shares) (7,500) 0 (13,950)
Ending Balance (in shares) 35,250 57,450 187,665
Executive Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Beginning Balance (in shares) 432,424 459,017 556,360
Grants (in shares) 153,000 167,500 179,500
Additional performance shares 106,084 (35,073) (58,056)
Restriction lapses (in shares) (119,896) (159,020) (211,287)
Forfeitures (in shares) (7,500) 0 (7,500)
Ending Balance (in shares) 564,112 432,424 459,017
Directors Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Beginning Balance (in shares) 1,894 0 0
Grants (in shares) 9,135 12,549 6,648
Additional performance shares
Restriction lapses (in shares) (11,029) (10,655) (6,648)
Forfeitures (in shares)
Ending Balance (in shares) 0 1,894 0
Directors Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Beginning Balance (in shares) 0 0 0
Grants (in shares) 7,735 6,912 7,640
Additional performance shares
Restriction lapses (in shares) (7,735) (6,912) (7,640)
Forfeitures (in shares)
Ending Balance (in shares) 0 0 0