Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Basic:
Net Income Attributable to BWX Technologies, Inc.	$ 73,171	$ 74,810	$ 212,928	$ 182,666
Weighted-average common shares (in shares)	95,483,003	95,420,626	95,450,994	95,344,349
Basic earnings per common share (in usd per share)	$ 0.77	$ 0.78	$ 2.23	$ 1.92
Diluted:
Net Income Attributable to BWX Technologies, Inc.	$ 73,171	$ 74,810	$ 212,928	$ 182,666
Weighted-average common shares (in shares)	95,483,003	95,420,626	95,450,994	95,344,349
Effect of dilutive securities:
Stock options, restricted stock units and performance shares (in shares)	247,693	390,572	255,886	425,570
Adjusted weighted-average common shares (in shares)	95,730,696	95,811,198	95,706,880	95,769,919
Diluted earnings per common share (in usd per share)	$ 0.76	$ 0.78	$ 2.22	$ 1.91