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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%