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, 2021	Mar. 31, 2020
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 40,611	$ 27,545
Weighted-average number of common shares outstanding		22,974	23,023
Basic earnings per share		$ 1.77	$ 1.20
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company		$ 40,611	$ 27,545
Weighted-average number of common shares outstanding		22,974	23,023
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		1	1
Weighted-average shares applicable to diluted earnings		23,330	23,285
Diluted earnings per share		$ 1.74	$ 1.18
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[1]	140	102
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]	171	128
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		44	31

[1] No options/SARs to acquire shares of Company common stock were excluded from the computation of dilutive earnings per share for the three months ended March 31, 2021. Options/SARs to acquire 256,288 shares of Company common stock were excluded from the computations of diluted earnings per share for the three months ended March 31, 2020. Inclusion of the instruments would have had an antidilutive effect on the computations of the earnings per share.