Accounting for Stock Based Compensation - Summarizes Restricted Stock Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding at beginning of period, Shares	34,920	136,566	282,100
Granted, Shares	32,406	19,920	21,681
Vested, Shares	(32,420)	(91,566)	(130,684)
Forfeited, Shares		(30,000)	(36,531)
Outstanding at end of period, Shares	34,906	34,920	136,566
Outstanding at beginning of period, Weighted-Average Grant Date Fair Value	$ 38.31	$ 37.16	$ 38.06
Granted, Weighted-Average Grant Date Fair Value	39.96	40.16	35.00
Vested, Weighted-Average Grant Date Fair Value	38.33	37.09	38.62
Forfeited, Weighted-Average Grant Date Fair Value		38.05	37.60
Outstanding at end of period, Weighted-Average Grant Date Fair Value	$ 39.82	$ 38.31	$ 37.16