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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 33,168	$ 25,889	$ 96,420	$ 81,091
Weighted-average number of common shares outstanding		22,907	23,025	22,951	23,070
Basic earnings per share		$ 1.45	$ 1.12	$ 4.20	$ 3.52
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company		$ 33,168	$ 25,889	$ 96,420	$ 81,091
Weighted-average number of common shares outstanding		22,907	23,025	22,951	23,070
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		1	1	1	2
Weighted-average shares applicable to diluted earnings		23,237	23,300	23,236	23,320
Diluted earnings per share		$ 1.43	$ 1.11	$ 4.15	$ 3.48
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[1]	119	102	107	95
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]	164	135	139	120
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		46	37	38	33

[1] Options/SARs to acquire 169,912 shares of Company common stock were excluded from the computations of diluted earnings per share for the nine months ended September 30, 2020. Options/SARs to acquire 142,833 shares of Company common stock were excluded from the computations of diluted earnings per share for the nine months ended September 30, 2019. Inclusion of the instruments would have had antidilutive effects on the computations of the earnings per share.