Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Net Sales	$ 466,659	$ 507,997	$ 519,866	$ 499,335	$ 473,823	$ 487,814	$ 495,101	$ 468,269	$ 1,993,857	[1]	$ 1,925,007	[1]	$ 1,766,166	[1]
Gross Profit	78,528	84,125	89,280	89,570	80,839	75,576	89,988	92,119	341,503	[2]	338,522	[2]	339,269	[2]
Operating income	39,385	27,694	44,685	39,655	31,398	30,434	39,133	46,230	151,419	[2]	147,195	[2]	127,830	[2]
Interest, net	(2,151)	(2,797)	(2,672)	(3,151)	(2,826)	(2,763)	(2,863)	(2,992)	(10,771)		(11,444)		(13,205)
Income Before Income Taxes	34,519	25,243	42,497	37,664	28,532	29,312	37,063	44,330	139,923		139,237		113,816
Net Income	26,943	22,168	32,923	30,716	9,886	21,853	27,896	31,912	112,750		91,547		86,198
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
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

[1] Net sales are attributed to countries based on the location of the Company facility making the sales.

[2] The 2017 and 2016 amounts for the noted line items have been immaterially changed from the amounts originally reported as a result of the Company’s first quarter 2018 adoption of ASU No. 2017-7, Compensation –Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost.