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Pension Plans and Postretirement Benefits - Additional Information (Detail) $ in Thousands | 1 Months Ended | 12 Months Ended
--- | --- | ---
Aug. 31, 2018 USD ($) | Jul. 31, 2018 USD ($) retiree | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Service cost |  |  | $ 11,300 | $ 9,700 | $ 10,200
Other components of net periodic benefit cost |  |  | (31,600) | $ (17,500) | 700
Company contributions |  |  | 5,131 |  |
Other comprehensive loss as components of net periodic benefit cost for pension benefit in the current fiscal year |  |  | $ 3,400 |  |
Percentage of investment return on domestic plan assets |  |  | 19.00% | 21.00% |
Vesting period for employer matching contributions |  |  | 3 years |  |
Other comprehensive income as components of net periodic benefit cost for other benefit in the current fiscal year |  |  | $ (200) |  |
BWXT Retirement Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Pension settlement related charges |  |  |  |  | 10,400
Thrift Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Percentage of matching contribution by employer |  |  | 50.00% |  |
Thrift Plan | Maximum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Employer matching percentage |  |  | 6.00% |  |
Thrift Plan and MII Thrift Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Employer contributions |  |  | $ 35,800 | $ 31,700 | 31,300
Pension Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Service cost |  |  | 10,650 | 9,123 | 9,593
Increase in pension plan benefit obligation |  |  | 124,200 | 156,800 |
Pension settlement related charges |  | $ 240,000 | $ 7,041 | 25,694 |
Number of retirees transferred to annuity contract | retiree |  | 1,300 |  |  |
Company contributions | $ 118,100 |  |  | $ 4,284 | $ 159,000
Expected return on plan assets |  |  | 6.95% | 6.95% | 6.95%

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