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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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