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Stock-Based Compensation - Summarized Activity of Stock Options (Detail) - BWXT Stock Options $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period (shares)	324
Granted (shares)	0
Exercised (shares)	(123)
Cancelled/expired/forfeited (shares)	0
Outstanding at end of period (shares)	201
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding at beginning of period (USD per share) | $ / shares	$ 23.91
Exercised (USD per share) | $ / shares	23.64
Outstanding at end of period (USD per share) | $ / shares	$ 24.07
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding at end of period, weighted-average remaining contractual term (in years)	2 years 10 months 24 days
Outstanding at end of period, aggregate intrinsic value | $	$ 7.3
Exercisable at end of period, Number of options (shares)	201
Exercisable at end of period, weighted-average exercise price (USD per share) | $ / shares	$ 24.07
Exercisable at end of period, weighted-average remaining contractual term	2 years 10 months 24 days
Exercisable at end of period, aggregate intrinsic value | $	$ 7.3

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Net number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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