Stock-Based Compensation - Summary of Restricted Stock and Restricted Stock Units Granted (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants (in shares)	171,974	177,493	169,870
Directors Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants (in shares)	8,840	10,805	9,135
Price per share (in dollars per share)	$ 76.37	$ 88.19	$ 73.92
Aggregate value	$ 675	$ 953	$ 675
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)	6,634	7,688	7,735
Price per share (in dollars per share)	$ 77.50	$ 89.15	$ 74.45
Aggregate value	$ 514	$ 685	$ 576
Percent vested	100.00%	100.00%	100.00%
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants (in shares)	156,500	159,000	153,000
Price per share (in dollars per share)	$ 82.56	$ 87.60	$ 77.26
Aggregate value	$ 12,921	$ 13,928	$ 11,821
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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