Incentive Compensation Plan - Unvested Target Award (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding, beginning of the year, Shares	171,500	137,500	117,000
Granted, Shares	95,300	78,000	84,500
Vested, Shares	(12,000)	(41,000)	(54,000)
Forfeited, Shares	(4,000)	(3,000)	(10,000)
Outstanding, end of the year, Shares	250,800	171,500	137,500
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding, beginning of the year, Shares	442,832	496,603	525,045
Granted, Shares	188,700	134,929	130,995
Vested, Shares	(134,452)	(188,700)	(149,480)
Forfeited, Shares	0	0	(9,957)
Outstanding, end of the year, Shares	497,080	442,832	496,603