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, 2019	Mar. 31, 2018
Computation of Basic Earnings per Share
Net income attributable to Stepan Company	[1],[2]	$ 24,984	$ 31,952
Weighted-average number of common shares outstanding		23,099	23,082
Basic earnings per share	[1],[2]	$ 1.08	$ 1.38
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company	[1]	$ 24,984	$ 31,952
Weighted-average number of common shares outstanding		23,099	23,082
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		3	2
Weighted-average shares applicable to diluted earnings		23,332	23,389
Net Income Per Diluted Common Share Attributable to Stepan Company	[1],[2]	$ 1.07	$ 1.37
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[3]	92	119
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]	108	122
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		30	64

[1] The 2018 amounts for the noted line items have been 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 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 214,241 and 2,807 shares of Company common stock were excluded from the computations of diluted earnings per share for the three months ended March 31, 2019 and March 31, 2018, because the effect of including the instruments would have been antidilutive.