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Stock Option Plan - Additional Information (Detail) |  |  |  |  | 12 Months Ended
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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 |  |  |  |  |

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