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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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Amount of tax benefit from exercise of option under share-based payment arrangement.

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Number of additional shares authorized for issuance under share-based payment arrangement.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Percentage of vesting of award under share-based payment arrangement.

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Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	May 31, 2020	Jun. 30, 2012	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2014	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense			$ 18.3	$ 16.8	$ 14.8
Stock-based compensation, tax benefits			$ 3.0	2.7	2.5
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			$ 2.7	$ 4.7	$ 8.1
Tax benefits realized related to the stock options exercised			0.3
BWXT Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefits realized related to the stock options exercised			$ 1.0
Nonvested cash-settled performance units (shares)			545,000	451,000
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			129.00%
BWXT Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefits realized related to the stock options exercised			$ 1.1
Nonvested cash-settled performance units (shares)			221,000	185,000
Cash Settled Performance Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested cash-settled performance units (shares)			3,973
Nonvested cash-settled performance units value per share			$ 47.88
Nonvested cash-settled performance units assumed weighted average vesting			128.00%
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)			1,715
Nonvested cash-settled performance units value per share			$ 47.88
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)			4,036,598
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			$ 14.8
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