Earnings Per Share - Computations of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 27,545	$ 24,984
Weighted-average number of common shares outstanding		23,023	23,099
Basic earnings per share		$ 1.20	$ 1.08
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company		$ 27,545	$ 24,984
Weighted-average number of common shares outstanding		23,023	23,099
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		1	3
Weighted-average shares applicable to diluted earnings		23,285	23,332
Diluted earnings per share		$ 1.18	$ 1.07
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[1]	102	92
Stock Appreciation Rights (SARs) [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[1]	128	108
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		31	30

[1] Options/SARs to acquire 256,288 and 214,241 shares of Company common stock were excluded from the computations of diluted earnings per share for the three months ended March 31, 2020 and March 31, 2019. Inclusion of the instruments would have had an antidilutive effect on the earnings per share computations.