Earnings Per Share - Computations of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 35,707	$ 30,218	$ 63,252	$ 55,202
Weighted-average number of common shares outstanding		22,923	23,086	22,973	23,092
Basic earnings per share		$ 1.56	$ 1.31	$ 2.75	$ 2.39
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company		$ 35,707	$ 30,218	$ 63,252	$ 55,202
Weighted-average number of common shares outstanding		22,923	23,086	22,973	23,092
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		1	2	1	2
Weighted-average shares applicable to diluted earnings		23,184	23,329	23,235	23,329
Diluted earnings per share		$ 1.54	$ 1.30	$ 2.72	$ 2.37
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[1]	100	90	101	91
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]	125	118	127	113
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		35	33	33	31

[1] Options/SARs to acquire 253,448 and 254,868 shares of Company common stock were excluded from the computations of diluted earnings per share for the three and six months ended June 30, 2020, respectively. Options/SARs to acquire 215,257 and 214,749 shares of Company common stock were excluded from the computations of diluted earnings per share for the three and six months ended June 30, 2019, respectively. Inclusion of the instruments would have had antidilutive effects on the computations of the earnings per share.