Stock Option Plan - Additional Information (Detail)					12 Months Ended
	Dec. 31, 2020 USD ($) $ / shares shares	Apr. 24, 2020 USD ($) person $ / shares	May 03, 2019 USD ($) person $ / shares shares	May 11, 2018 USD ($) person $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 $ / shares	Dec. 31, 2018 $ / shares	Dec. 31, 2004 shares
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Deferred compensation (in shares)	118,628				11,574
Exercise price of options (in usd per share) | $ / shares					$ 50.00	$ 55.71	$ 49.46
Closing price (in usd per share) | $ / shares	$ 31.68				$ 31.68
Allocated share based compensation | $					$ 1,200,000
Future expense | $	$ 1,600,000				$ 1,600,000
2017 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price of options (in usd per share) | $ / shares				$ 49.46
Fair value stock option grant | $				$ 1,400,000
2018 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price of options (in usd per share) | $ / shares			$ 55.71
Fair value stock option grant | $			$ 1,900,000
2019 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price of options (in usd per share) | $ / shares		$ 50.00
Closing price (in usd per share) | $ / shares		$ 28.02
Fair value stock option grant | $		$ 200,000
Common Stock
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares)					7,354
Officer | 2017 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Number of individuals granted options | person				22
Options vested				25.00%
Fair value stock option grant | $				$ 1,200,000
Officer | 2018 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Number of individuals granted options | person			23
Options vested			25.00%
Fair value stock option grant | $			$ 1,700,000
Officer | 2019 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Number of individuals granted options | person		20
Options vested		25.00%
Fair value stock option grant | $		$ 200,000
Director | 2017 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Number of individuals granted options | person				11
Fair value stock option grant | $				$ 169,400
Director | 2018 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Number of individuals granted options | person			11
Fair value stock option grant | $			$ 226,600
Director | 2019 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Number of individuals granted options | person		11
Fair value stock option grant | $		$ 23,100
Stock options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Expiration period								10 years
Stock options | Amended 2004 Plan
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Authorized shares for a grant (in shares)								3,400,000
Stock options | 2017 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options granted to purchase stocks (in shares)				245,000
Stock options | 2018 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options granted to purchase stocks (in shares)			260,000
Stock options | 2019 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options granted to purchase stocks (in shares)			238,000
Stock options | Officer
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Option vesting period								4 years
Stock options | Officer | 2017 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Option vesting period				4 years
Stock options | Officer | 2018 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Option vesting period			4 years
Stock options | Officer | 2019 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Option vesting period		4 years
Incentive stock option | 2017 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options granted to purchase stocks (in shares)				25,914
Incentive stock option | 2018 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options granted to purchase stocks (in shares)			34,651
Incentive stock option | 2019 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options granted to purchase stocks (in shares)			29,624
Nonqualified stock options | 2017 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options granted to purchase stocks (in shares)				219,086
Nonqualified stock options | 2018 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options granted to purchase stocks (in shares)			225,349
Nonqualified stock options | 2019 Options
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options granted to purchase stocks (in shares)			208,376