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, 2018	Mar. 31, 2017
Computation of Basic Earnings per Share
Net income attributable to Stepan Company	[1]	$ 30,723	$ 31,913
Weighted-average number of common shares outstanding		23,082	22,901
Basic earnings per share		$ 1.33	$ 1.39
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company		$ 30,723	$ 31,913
Weighted-average number of common shares outstanding		23,082	22,901
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[2]	119	183
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		2	8
Weighted-average shares applicable to diluted earnings		23,389	23,331
Diluted earnings per share		$ 1.31	$ 1.37
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)	[2]	122	142
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		64	97

[1] The 2017 amounts for the noted line items have been immaterially changed from the amounts originally reported as a result of the Company’s first quarter 2018 adoption of Accounting Standards Update (ASU) No. 2017-7, Compensation –Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost.

[2] Options/SARs to acquire 2,807 and 74,521 shares of Company common stock were excluded from the computations of diluted earnings per share for the three months ended March 31, 2018 and March 31, 2017, because the effect of including the instruments would have been antidilutive.