Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended												12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 22,038	$ 25,889	$ 30,218	$ 24,984	$ 23,957		$ 21,754		$ 33,454		$ 31,952		$ 103,129	[2]	$ 111,117	[1],[2]	$ 100,774	[2]
Weighted-average number of shares outstanding														23,054		23,022		22,946
Basic earnings per share	[2]													$ 4.47		$ 4.83		$ 4.39
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company	[2]													$ 103,129		$ 111,117		$ 100,774
Weighted-average number of shares outstanding														23,054		23,022		22,946
Weighted-average shares applicable to diluted earnings														23,316		23,325		23,377
Net Income Per Diluted Common Share Attributable to Stepan Company		$ 0.95	$ 1.11	$ 1.30	$ 1.07	$ 1.03		$ 0.93		$ 1.44		$ 1.37		$ 4.42	[2]	$ 4.76	[1],[2]	$ 4.31	[2]
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury share method)	[3]													97		101		161
Unvested Stock Awards [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares related to unvested stock awards (under treasury share method)														2		3		8
Stock Appreciation Rights (SARs) [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury share method)														125		110		142
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury share method)														38		89		120

[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 and 2017 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.

[3] Options/SARs to purchase 107,125, 50,770 and 18,630 shares of common stock were not included in the computations of diluted earnings per share for the years ended December 31, 2019, 2018 and 2017, respectively. The options’/SARs’ exercise prices were greater than the average market price for the common stock and the effect of the options/SARs on earnings per share would have been antidilutive.