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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Computation of Basic Earnings per Share
Net income attributable to Stepan Company	[1],[2]	$ 25,889	$ 21,754	$ 81,091	$ 87,160
Weighted-average number of common shares outstanding		23,025	22,986	23,070	23,036
Basic earnings per share	[1],[2]	$ 1.12	$ 0.95	$ 3.52	$ 3.78
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company	[1]	$ 25,889	$ 21,754	$ 81,091	$ 87,160
Weighted-average number of common shares outstanding		23,025	22,986	23,070	23,036
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		1	3	2	2
Weighted-average shares applicable to diluted earnings		23,300	23,288	23,320	23,324
Net Income Per Diluted Common Share Attributable to Stepan Company	[1],[2]	$ 1.11	$ 0.93	$ 3.48	$ 3.74
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[3]	102	103	95	103
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)	[3]	135	113	120	111
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		37	83	33	72

[1] The 2018 amounts for the noted line items have been retrospectively changed from the amounts originally reported as a result of the Company’s first quarter 2019 change in method of accounting for U.S. inventory valuation from LIFO to FIFO.

[2] The 2018 amounts for the noted line items have been retrospectively changed from the amounts originally reported as a result of the Company’s first quarter 2019 change in method of accounting for U.S. inventory valuation from the last in, first out (LIFO) basis to the first in, first out (FIFO) basis.

[3] Options/SARs to acquire 142,833 shares of Company common stock were excluded from the computations of diluted earnings per share for the nine months ended September 30, 2019.  Options/SARs to acquire 65,919 shares of Company common stock were excluded from the computations of diluted earnings per share for the nine months ended September 30, 2018.  Inclusion of the instruments would have had antidilutive effects on the computations of the earnings per share.