Stock Plans - Summary of Restricted Stock Activity (Detail) - Restricted stock - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Shares
Beginning balance (in shares)	146,191	172,701	205,449
Granted (in shares)	43,146	51,468	70,160
Vested (in shares)	(65,225)	(70,399)	(97,253)
Forfeited (in shares)	(5,769)	(7,579)	(5,655)
Ending balance (in shares)	118,343	146,191	172,701
Weighted Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 105.83	$ 90.38	$ 68.97
Granted (in dollars per share)	167.13	124.61	104.53
Vested (in dollars per share)	95.43	81.58	56.47
Forfeited (in dollars per share)	132.54	106.54	85.48
Ending balance (in dollars per share)	$ 132.62	$ 105.83	$ 90.38