Earnings Per Share - Reconciliation of the number of shares (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Earnings Per Share [Abstract]
Net income attributable to Apergy		$ (1,823)	$ 11,394	$ 22,937	$ 19,656	$ 23,187	$ 23,961	$ 21,816	$ 23,773	$ 52,164		$ 92,737		$ 109,589
Weighted-average number of shares outstanding (in shares)	[1]									77,427		77,342		77,340
Dilutive effect of stock-based compensation (in shares)										197		350		550
Total shares and dilutive securities (in shares)	[1]									77,624		77,692		77,890
Basic earnings per share attributable to Apergy (in dollars per share)		$ (0.02)	$ 0.15	$ 0.30	$ 0.25	$ 0.30	$ 0.31	$ 0.28	$ 0.31	$ 0.67	[1]	$ 1.20	[1]	$ 1.42	[1]
Diluted earnings per share attributable to Apergy (in dollars per share)		$ (0.02)	$ 0.15	$ 0.30	$ 0.25	$ 0.30	$ 0.31	$ 0.28	$ 0.31	$ 0.67	[1]	$ 1.19	[1]	$ 1.41	[1]

[1] See Note 4—Earnings Per Share.