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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Computation of Basic Earnings per Share
Net income attributable to Stepan Company	[1],[2]	$ 30,218	$ 33,454	$ 55,202	$ 65,406
Weighted-average number of common shares outstanding		23,086	23,039	23,092	23,059
Basic earnings per share	[1],[2]	$ 1.31	$ 1.45	$ 2.39	$ 2.84
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company	[1]	$ 30,218	$ 33,454	$ 55,202	$ 65,406
Weighted-average number of common shares outstanding		23,086	23,039	23,092	23,059
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		2	2	2	2
Weighted-average shares applicable to diluted earnings		23,329	23,295	23,329	23,341
Net Income Per Diluted Common Share Attributable to Stepan Company	[1],[2]	$ 1.30	$ 1.44	$ 2.37	$ 2.80
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[3]	90	87	91	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]	118	99	113	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)		33	68	31	66

[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 215,257 and 214,749 shares of Company common stock were excluded from the computations of diluted earnings per share for the three and six months ended June 30, 2019, respectively. Options/SARs to acquire 194,949 and 98,878 shares of Company common stock were excluded from the computations of diluted earnings per share for the three and six months ended June 30, 2018, respectively. Inclusion of the instruments would have had antidilutive effects on the computations of the earnings per share.