Stock-Based Compensation - Summary of Restricted Stock and Restricted Stock Units Granted (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants (in shares)	161,909	171,974	177,493
Directors Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants (in shares)	4,548	8,840	10,805
Price per share (in dollars per share)	$ 105.56	$ 76.37	$ 88.19
Aggregate value	$ 480	$ 675	$ 953
Percent vested	100.00%	100.00%	100.00%
Directors Restricted Stock Units Including Dividend Equivalents
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants (in shares)	6,161	6,634	7,688
Price per share (in dollars per share)	$ 103.32	$ 77.50	$ 89.15
Aggregate value	$ 637	$ 514	$ 685
Percent vested	100.00%	100.00%	100.00%
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants (in shares)	151,200	156,500	159,000
Price per share (in dollars per share)	$ 100.74	$ 82.56	$ 87.60
Aggregate value	$ 15,232	$ 12,921	$ 13,928
Percent vested	0.00%	0.00%	0.00%

Restricted Stock And Restricted Stock Units Granted, Vested Percentage

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Total number of shares issued during the period, including shares forfeited, as a result of Restricted Stock Awards.

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Aggregate value of stock related to Restricted Stock Awards issued during the period.

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