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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 43,278	$ 35,707	$ 83,889	$ 63,252
Weighted-average number of common shares outstanding		22,952	22,923	22,963	22,973
Basic earnings per share		$ 1.89	$ 1.56	$ 3.65	$ 2.75
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company		$ 43,278	$ 35,707	$ 83,889	$ 63,252
Weighted-average number of common shares outstanding		22,952	22,923	22,963	22,973
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		1	1	1	1
Weighted-average shares applicable to diluted earnings		23,345	23,184	23,338	23,235
Diluted earnings per share		$ 1.85	$ 1.54	$ 3.59	$ 2.72
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[1]	152	100	146	101
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]	190	125	181	127
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		50	35	47	33

[1] No options/SARs to acquire shares of Company common stock were excluded from the computations of diluted earnings per share for the three and six months ended June 30, 2021, respectively. 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. Inclusion of the instruments would have had an antidilutive effect on the computations of the earnings per share.