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Employee Benefits - Additional Information (Detail) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage of company stock held equal to plan assets | 12.00% |  |
Period of obligations for retired lives in trust | 30 years |  |
Long term inflation rate | 2.50% |  |
Company's contribution | $ 5,700,000 | $ 0 |
Contribution to Health & Welfare Plan to pay premiums and claims in the next fiscal year | $ 915,000 |  |
Minimum |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Expected rate of return on equity security | 6.00% |  |
Expected rate of return on fixed income security | 3.00% |  |
Overall expected rate of return | 5.00% |  |
Maximum |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Expected rate of return on equity security | 8.00% |  |
Expected rate of return on fixed income security | 5.00% |  |
Overall expected rate of return | 7.00% |  |
Pension Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Estimated unrecognized net actuarial loss (gain) that will be amortized from accumulated other comprehensive loss into net periodic benefit cost | $ 6,900 |  |
Amount recognized of net actuarial loss into net periodic benefit cost | 7,327,000 | $ 10,035,000 | $ 7,179,000
Estimated prior service cost that will be amortized from accumulated other comprehensive loss (gain) into net periodic benefit cost | 0 |  |
Amortization of past-service liability | $ 0 |  |
Discount rates used to determine the benefit obligation | 2.20% | 3.00% |
Percentage of assets allocated to equities to be considered well funded | 55.00% |  |
Percentage of assets allocated to fixed income to be considered well funded | 44.00% |  |
Defined benefit plan cash equivalent portfolio percentage | 1.00% |  |
Post-Retirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Estimated unrecognized net actuarial loss (gain) that will be amortized from accumulated other comprehensive loss into net periodic benefit cost | $ 46,000 | $ 249,000 |
Amount recognized of net actuarial loss into net periodic benefit cost | 25,000 | 124,000 | 309,000
Amortization of past-service liability | $ (249,000) | $ (249,000) | $ (249,000)
Discount rates used to determine the benefit obligation | 2.00% | 2.90% |

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