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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 26,946	$ 22,168	$ 32,925	$ 30,723	$ 9,884	$ 21,899	$ 27,882	$ 31,913	$ 112,762	$ 91,578	$ 86,191
Weighted-average number of shares outstanding										23,022	22,946	22,793
Basic earnings per share										$ 4.90	$ 3.99	$ 3.78
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company										$ 112,762	$ 91,578	$ 86,191
Weighted-average number of shares outstanding										23,022	22,946	22,793
Add weighted-average net shares related to unvested stock awards (under treasury share method)										3	8	6
Weighted-average shares applicable to diluted earnings										23,325	23,377	23,094
Diluted earnings per share		$ 1.16	$ 0.95	$ 1.41	$ 1.31	$ 0.42	$ 0.94	$ 1.19	$ 1.37	$ 4.83	$ 3.92	$ 3.73
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury share method)	[1]									101	161	159
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)										110	142	68
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury share method)										89	120	68

[1] Options/SARs to purchase 50,770, 18,630 and 43,715 shares of common stock were not included in the computations of diluted earnings per share for the years ended December 31, 2018, 2017 and 2016, 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.