Restricted Stock Units Issued (Detail) - Restricted Stock Units (RSUs) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restricted Stock Units
Beginning Restricted Stock Units	70,364	61,789
Restricted Stock Units Granted	3,743	6,943
Dividend Equivalents Earned	1,507	1,632
Restricted Stock Units Settled	(32,422)
Ending Restricted Stock Units	43,192	70,364
Weighted-Average Stock Price
Beginning Restricted Stock Units	$ 41.20	$ 38.25
Restricted Stock Units Granted	39.26	63.50
Dividend Equivalents Earned	47.34	58.15
Restricted Stock Units Settled	41.09
Ending Restricted Stock Units	$ 41.34	$ 41.20