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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Computation of Basic Earnings per Share
Net income attributable to Stepan Company		$ 32,925	$ 27,882	$ 63,648	$ 59,795
Weighted-average number of common shares outstanding		23,039	22,953	23,059	22,927
Basic earnings per share		$ 1.43	$ 1.21	$ 2.76	$ 2.61
Computation of Diluted Earnings per Share
Net income attributable to Stepan Company		$ 32,925	$ 27,882	$ 63,648	$ 59,795
Weighted-average number of common shares outstanding		23,039	22,953	23,059	22,927
Add weighted-average net shares from assumed exercise of options (under treasury stock method)	[1]	87	172	103	177
Add weighted-average net shares related to unvested stock awards (under treasury stock method)		2	8	2	8
Weighted-average shares applicable to diluted earnings		23,295	23,381	23,341	23,356
Diluted earnings per share		$ 1.41	$ 1.19	$ 2.73	$ 2.56
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)	[1]	99	143	111	143
Performance Stock Award [Member]
Computation of Diluted Earnings per Share
Add weighted-average net shares from assumed exercise of options (under treasury stock method)		68	105	66	101

[1] 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. Options/SARs to acquire 37,261 shares of Company common stock were excluded from the computations of diluted earnings per share for the six months ended June 30, 2017. Inclusion of the instruments would have had antidilutive effects on the computations of the earnings per share.