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Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
May 31, 2020 | Jun. 30, 2013 | Jun. 30, 2012 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock based compensation expense |  |  |  | $ 16.8 | $ 14.8 | $ 10.8 |  |  |
Stock-based compensation, tax benefits |  |  |  | $ 2.7 | 2.5 | 1.7 |  |  |
Performance period |  |  |  | 3 years |  |  |  |  |
BWXT Stock Options |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Total intrinsic value of stock options exercised |  |  |  | $ 4.7 | $ 8.1 | $ 8.5 |  |  |
Tax benefits realized related to the stock options exercised |  |  |  | 0.8 |  |  |  |  |
BWXT Performance Shares |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Tax benefits realized related to the stock options exercised |  |  |  | $ 1.4 |  |  |  |  |
Nonvested cash-settled performance units (shares) |  |  |  | 451,000 | 409,000 |  |  |  |
BWXT Performance Shares | Minimum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award vesting rights, percentage |  |  |  | 0.00% |  |  |  |  |
BWXT Performance Shares | Maximum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award vesting rights, percentage |  |  |  | 200.00% |  |  |  |  |
BWXT Performance Shares | Weighted Average |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award vesting rights, percentage |  |  |  | 121.00% |  |  |  |  |
BWXT Restricted Stock Units |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Tax benefits realized related to the stock options exercised |  |  |  | $ 1.3 |  |  |  |  |
Nonvested cash-settled performance units (shares) |  |  |  | 185,000 | 166,000 |  |  |  |
Cash Settled Performance Units |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Nonvested cash-settled performance units (shares) |  |  |  | 3,453 |  |  |  |  |
Nonvested cash-settled performance units value per share |  |  |  | $ 60.28 |  |  |  |  |
Nonvested cash-settled performance units assumed weighted average vesting |  |  |  | 104.00% |  |  |  |  |
Cash Settled Performance Units | Minimum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award vesting rights, percentage |  |  |  | 0.00% |  |  |  |  |
Cash Settled Performance Units | Maximum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award vesting rights, percentage |  |  |  | 200.00% |  |  |  |  |
Cash Settled Restricted Stock Units |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Nonvested cash-settled performance units (shares) |  |  |  | 1,422 |  |  |  |  |
Nonvested cash-settled performance units value per share |  |  |  | $ 60.28 |  |  |  |  |
2010 Long-Term Incentive Plan |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Additional shares authorized for issuance (shares) |  |  |  |  |  |  |  | 2,300,000 |
Number of shares authorized for issuance (shares) |  |  |  |  |  |  |  |  | 10,000,000
Minimum percentage of fair market value closing price |  |  |  |  |  |  |  | 100.00% |
Expiry date of common stock grants |  |  |  |  |  |  | 10 years |  |
Shares available for issuance of awards in future (shares) |  |  |  | 4,333,710 |  |  |  |  |
2012 Long-Term Incentive Plan |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Minimum percentage of fair market value closing price |  |  | 100.00% |  |  |  |  |  |
Expiry date of common stock grants |  | 10 years |  |  |  |  |  |  |
Total unrecognized estimated compensation expense, nonvested awards |  |  |  | $ 15.1 |  |  |  |  |
Unrecognized estimated compensation expense to be recognized, weighted-average period |  |  |  | 1 year 7 months 6 days |  |  |  |  |
2020 Plan |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Additional shares authorized for issuance (shares) | 1,450,000 |  |  |  |  |  |  |  |
Minimum percentage of fair market value closing price | 100.00% |  |  |  |  |  |  |  |
Expiry date of common stock grants | 10 years |  |  |  |  |  |  |  |

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