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Selected Balance Sheet Data - Additional Information (Detail) - USD ($) |  |  |  | 3 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 01, 2021 | Jan. 01, 2020 | Jan. 01, 2014 | Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020 | Mar. 31, 2013
Schedule Of Accrued Expenses [Line Items] |  |  |  |  |  |  |
SARs frozen liability amount |  |  |  | $ 14,568,000 |  | $ 16,671,000 |
Interest expense |  |  |  | $ 146,000 | $ 283,000 |  |
Commissions Payable [Member] |  |  |  |  |  |  |
Schedule Of Accrued Expenses [Line Items] |  |  |  |  |  |  |
Estimated payouts description |  |  |  | Commissions payable that are not expected to be paid within twelve months are classified as long-term. |  |  |
Maximum payment deferral period for certain commissions payable |  |  |  | 3 years |  |  |
Mortgage servicing rights [Member] |  |  |  |  |  |  |
Schedule Of Accrued Expenses [Line Items] |  |  |  |  |  |  |
Servicing portfolio of commercial real estate loans, unpaid principal balance |  |  |  | $ 1,600,000,000 |  | 1,600,000,000 |
Escrow Funds |  |  |  | $ 2,000,000.0 |  | $ 3,200,000 |
SARs [Member] |  |  |  |  |  |  |
Schedule Of Accrued Expenses [Line Items] |  |  |  |  |  |  |
SARs frozen liability amount |  |  |  |  |  |  | $ 20,000,000.0
SARs liability frozen value date |  |  |  | Mar. 31, 2013 |  |  |
SARs liability interest accrual commencement date |  |  |  | Jan. 01, 2014 |  |  |
SARs liability interest accrual rates | 2.93% | 3.92% |  |  |  |  |
Interest expense |  |  |  | $ 122,000 | 178,000 |  |
Treasury note term |  |  | 10 years |  |  |  |
Base spread on SARs liability variable rate |  |  | 2.00% |  |  |  |
Estimated payouts description |  |  |  | Estimated payouts within the next twelve months for participants that have separated from service have been classified as current. |  |  |
Payments made during the period |  |  |  | $ 2,200,000 | 2,100,000 |  |
Deferred Compensation Liability [Member] |  |  |  |  |  |  |
Schedule Of Accrued Expenses [Line Items] |  |  |  |  |  |  |
Estimated payouts description |  |  |  | Estimated payouts within the next twelve months for participants that have separated from service or elected in service payout have been classified as current. |  |  |
Deferred Compensation Liability, Minimum Payout Period |  |  |  | 2 years |  |  |
Deferred Compensation Liability, Maximum Payout Period |  |  |  | 15 years |  |  |
Fair value of deferred compensation plan assets |  |  |  | 110.00% |  |  |
Payments made during the period |  |  |  | $ 371,000 | $ 358,000 |  |

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