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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 9,884	$ 21,899	$ 27,882	$ 31,913	$ 8,417	$ 21,362	$ 28,496	$ 27,916	$ 91,578	$ 86,191	$ 75,968
Weighted-average number of shares outstanding										22,946	22,793	22,730
Basic earnings per share										$ 3.99	$ 3.78	$ 3.34
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company										$ 91,578	$ 86,191	$ 75,968
Weighted-average number of shares outstanding										22,946	22,793	22,730
Add weighted-average net shares related to unvested stock awards (under treasury share method)										8	6	3
Weighted-average shares applicable to diluted earnings										23,377	23,094	22,858
Diluted earnings per share		$ 0.42	$ 0.94	$ 1.19	$ 1.37	$ 0.36	$ 0.92	$ 1.24	$ 1.22	$ 3.92	$ 3.73	$ 3.32
Stock Option [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options, SARS and contingently issuable net shares related to performance stock awards (under treasury share method)	[1]									161	159	118
Stock Appreciation Rights (SARs) [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options, SARS and contingently issuable net shares related to performance stock awards (under treasury share method)										142	68	1
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options, SARS and contingently issuable net shares related to performance stock awards (under treasury share method)										120	68	6

[1] Options to purchase 18,630, 43,715 and 124,531 shares of common stock were not included in the computations of diluted earnings per share for the years ended December 31, 2017, 2016 and 2015, respectively. The options’ exercise prices were greater than the average market price for the common stock and the effect of the options on earnings per share would have been antidilutive.