Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic
Net income attributable to BWX Technologies, Inc.	$ 66,441	$ 55,719
Weighted-average common shares (in shares)	99,526,187	99,444,910
Basic earnings per common share (in usd per share)	$ 0.67	$ 0.56
Diluted
Net income attributable to BWX Technologies, Inc.	$ 66,441	$ 55,719
Weighted-average common shares (in shares)	99,526,187	99,444,910
Effect of dilutive securities
Stock options, restricted stock units and performance shares (in shares)	986,100	1,246,058
Adjusted weighted-average common shares (in shares)	100,512,287	100,690,968
Diluted earnings per common share (in usd per share)	$ 0.66	$ 0.55
Antidilutive shares related to stock options excluded from the diluted share (in shares)	79,278	103,608