Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic:
Net Income Attributable to BWX Technologies, Inc.	$ 64,258	$ 58,878	$ 139,757	$ 107,856
Weighted-average common shares (in shares)	95,457,629	95,357,311	95,434,990	95,306,210
Basic earnings per common share (in usd per share)	$ 0.67	$ 0.62	$ 1.46	$ 1.13
Diluted:
Net Income Attributable to BWX Technologies, Inc.	$ 64,258	$ 58,878	$ 139,757	$ 107,856
Weighted-average common shares (in shares)	95,457,629	95,357,311	95,434,990	95,306,210
Effect of dilutive securities:
Stock options, restricted stock units and performance shares (in shares)	175,942	319,893	259,982	443,070
Adjusted weighted-average common shares (in shares)	95,633,571	95,677,204	95,694,972	95,749,280
Diluted earnings per common share (in usd per share)	$ 0.67	$ 0.62	$ 1.46	$ 1.13
Number of shares that would be antidilutive (in shares)			8,321	52,386