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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Computation of Basic Earnings per Share
Net income attributable to Stepan Company	[1]	$ 21,899	$ 21,362	$ 81,694	$ 77,774
Weighted-average number of common shares outstanding		22,971	22,819	22,941	22,771
Basic earnings per share	[1]	$ 0.95	$ 0.94	$ 3.56	$ 3.42
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company		$ 21,899	$ 21,362	$ 81,694	$ 77,774
Weighted-average number of common shares outstanding		22,971	22,819	22,941	22,771
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		8	7	8	6
Weighted-average shares applicable to diluted earnings	[1]	23,374	23,148	23,361	23,017
Diluted earnings per share	[1]	$ 0.94	$ 0.92	$ 3.50	$ 3.38
Options [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares issuable from assumed exercise of options, SARS and contingently issuable net shares related to performance stock awards (under treasury stock method)	[2]	147	162	167	151
Stock Appreciation Rights (SARs) [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares issuable from assumed exercise of options, SARS and contingently issuable net shares related to performance stock awards (under treasury stock method)	[2]	140	93	142	45
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares issuable from assumed exercise of options, SARS and contingently issuable net shares related to performance stock awards (under treasury stock method)		108	67	103	44

[1] The 2016 amounts for the noted items have been immaterially changed from the amounts originally reported as a result of the Company’s fourth quarter 2016 adoption of Accounting Standards Update (ASU) No. 2016-9, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting.

[2] Options/SARs to acquire 24,840 shares of Company common stock were excluded from the computations of diluted earnings per share for the nine months ended September 30, 2017. Options/SARs to acquire 58,287 shares of Company common stock were excluded from the computations of diluted earnings per share for the nine months ended September 30, 2016. Inclusion of the instruments would have had antidilutive effects on the computations of earnings per share.