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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 36,920	$ 33,168	$ 120,809	$ 96,420
Weighted-average number of common shares outstanding		22,898	22,907	22,941	22,951
Basic earnings per share		$ 1.61	$ 1.45	$ 5.27	$ 4.20
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company		$ 36,920	$ 33,168	$ 120,809	$ 96,420
Weighted-average number of common shares outstanding		22,898	22,907	22,941	22,951
Add weighted-average net shares related to unvested stock awards (under treasury stock method)			1	1	1
Weighted-average shares applicable to diluted earnings		23,219	23,237	23,299	23,236
Diluted earnings per share		$ 1.59	$ 1.43	$ 5.19	$ 4.15
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[1]	124	119	139	107
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]	142	164	168	139
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		55	46	50	38

[1] 69,199 and 207,598 options/SARs to acquire shares of Company common stock were excluded from the computations of diluted earnings per share for the three and nine months ended September 30, 2021, respectively. Options/SARs to acquire 169,912 shares of Company common stock were excluded from the computation of diluted earnings per share for the nine months ended September 30, 2020. Inclusion of the instruments would have had an antidilutive effect on the computation of diluted earnings per share.