Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Net Sales	$ 444,990	$ 451,582	$ 473,003	$ 489,170	$ 466,659		$ 507,997		$ 519,866		$ 499,335		$ 1,858,745	[1]	$ 1,993,857	[1]	$ 1,925,007	[1]
Gross Profit	84,744	77,402	92,959	84,609	74,594	[2]	83,576	[2]	89,981	[2]	91,198	[2]	339,714	[3]	339,349	[2],[3]	346,167	[3]
Operating Income	28,491	27,966	41,065	29,738	35,451	[2]	27,145	[2]	45,386	[2]	41,283	[2]	127,260	[3]	149,265	[2],[3]	154,840	[3]
Interest, net	(911)	(1,402)	(1,766)	(1,853)	(2,151)		(2,797)		(2,672)		(3,151)		(5,932)		(10,771)		(11,444)
Income Before Income Taxes	27,886	27,449	39,534	31,030	30,585	[2]	24,694	[2]	43,198	[2]	39,292	[2]	125,899	[3],[4]	137,769	[2],[3],[4]	146,882	[3],[4]
Net Income	22,033	25,880	30,210	24,978	23,954	[2]	21,754	[2]	33,452	[2]	31,945	[2]	103,101	[3]	111,105	[2],[3],[5]	100,743	[3],[5]
Net Income Attributable to Stepan Company	$ 22,038	$ 25,889	$ 30,218	$ 24,984	$ 23,957	[2]	$ 21,754	[2]	$ 33,454	[2]	$ 31,952	[2]	$ 103,129	[3]	$ 111,117	[2],[3]	$ 100,774	[3]
Per Diluted Share	$ 0.95	$ 1.11	$ 1.30	$ 1.07	$ 1.03	[2]	$ 0.93	[2]	$ 1.44	[2]	$ 1.37	[2]	$ 4.42	[3]	$ 4.76	[2],[3]	$ 4.31	[3]

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

[2] 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

[3] 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.

[4] 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

[5] The retained earnings and net income 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.