Earnings Per Share - Computations of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 22,168	$ 21,899	$ 85,816	$ 81,694
Weighted-average number of common shares outstanding		22,986	22,971	23,036	22,941
Basic earnings per share		$ 0.96	$ 0.95	$ 3.73	$ 3.56
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company		$ 22,168	$ 21,899	$ 85,816	$ 81,694
Weighted-average number of common shares outstanding		22,986	22,971	23,036	22,941
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[1]	103	147	103	167
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		3	8	2	8
Weighted-average shares applicable to diluted earnings		23,288	23,374	23,324	23,361
Diluted earnings per share		$ 0.95	$ 0.94	$ 3.68	$ 3.50
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]	113	140	111	142
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		83	108	72	103

[1] Options/SARs to acquire 65,919 shares of Company common stock were excluded from the computations of diluted earnings per share for the nine months ended September 30, 2018. Options/SARs to acquire 24,840 shares of Company common stock were excluded from the computations of diluted earnings per share for the nine months ended September 30, 2017. Inclusion of the instruments would have had antidilutive effects on the computations of the earnings per share.