Employee Benefit Plans - Summary of Restricted Shares Issued, Vested and Forfeited (Detail)	12 Months Ended
	Dec. 31, 2020 Employee shares	Dec. 31, 2019 Employee shares	Dec. 31, 2018 Employee shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock awards, shares issued	37,382	38,100	33,000
Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock awards, shares issued	37,382	38,100	33,000
Restricted shares vested	23,491	26,070	28,790
Restricted shares awarded due to retirement	10,890	14,300
Restricted shares awarded for other	1,050
Restricted shares forfeited	2,175	3,220	2,620
Restricted shares issued | Employee	92	94	80
Restricted shares vested | Employee	57	63	56
Restricted shares awarded due to retirement | Employee	8	3
Restricted shares awarded for other | Employee	1
Restricted shares forfeited | Employee	5	4	5