Change in Method of Accounting for Inventory Valuation - Summary of Current Year Financial Statement Line Items If Company Does Not Changed Method of Accounting for U.S. Inventories from LIFO to FIFO (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018		Sep. 30, 2019	Sep. 30, 2018		Dec. 31, 2018
Income Statement [Abstract]
Cost of Sales	[1]	$ 374,180	$ 424,421		$ 1,158,785	$ 1,262,443
Gross Profit	[1]	77,402	83,576		254,970	264,755
Operating Income	[1],[2]	27,966	27,145		98,769	113,814
Income Before Provision for Income Taxes	[1],[2]	27,449	24,694		98,013	107,184
Provision for Income Taxes	[1]	1,569	2,940		16,945	20,033
Net Income	[1],[2]	25,880	21,754	[3]	81,068	87,151	[3]
Net Income Attributable to Stepan Company	[1],[2]	$ 25,889	$ 21,754		$ 81,091	$ 87,160
Net Income Per Diluted Common Share Attributable to Stepan Company	[1],[2]	$ 1.11	$ 0.93		$ 3.48	$ 3.74
Balance Sheet [Abstract]
Inventories	[2]	$ 203,303			$ 203,303			$ 231,528
Other Non-Current Assets	[2]	10,985			10,985			9,549
Total Assets	[2]	1,518,370			1,518,370			1,514,614
Deferred Income Taxes	[2]	25,218			25,218			24,961
Retained Earnings	[2],[4]	906,612			906,612			837,107
Total Liabilities and Equity	[2]	1,518,370			1,518,370			1,514,614
Statement of Cash Flows [Abstract]
Net Income	[1],[2]	25,880	$ 21,754	[3]	81,068	$ 87,151	[3]
Deferred Income Taxes	[2]				(2,291)	6,846
Changes in assets and liabilities:
Inventories	[2]				25,879	(23,366)
Effect of Change [Member]
Income Statement [Abstract]
Cost of Sales		(1,000)	549		(3,000)	(1,780)
Gross Profit		1,000	(549)		3,000	1,780
Operating Income		1,000	(549)		3,000	1,780
Income Before Provision for Income Taxes		1,000	(549)		3,000	1,780
Provision for Income Taxes		246	(135)		738	436
Net Income		754	(414)		2,262	1,344
Net Income Attributable to Stepan Company		$ 754	$ (414)		$ 2,262	$ 1,344
Net Income Per Diluted Common Share Attributable to Stepan Company		$ 0.03	$ (0.02)		$ 0.10	$ 0.06
Balance Sheet [Abstract]
Inventories		$ (28,363)			$ (28,363)			31,363
Other Non-Current Assets		1,415			1,415			(1,415)
Total Assets		(26,948)			(26,948)			29,948
Deferred Income Taxes		(5,551)			(5,551)			6,289
Retained Earnings		(21,397)			(21,397)			23,659
Total Liabilities and Equity		(26,948)			(26,948)			$ 29,948
Statement of Cash Flows [Abstract]
Net Income		754	$ (414)		2,262	$ 1,344
Deferred Income Taxes					738	436
Changes in assets and liabilities:
Inventories					(3,000)	$ (1,780)
As Computed under LIFO [Member]
Income Statement [Abstract]
Cost of Sales		373,180			1,155,785
Gross Profit		78,402			257,970
Operating Income		28,966			101,769
Income Before Provision for Income Taxes		28,449			101,013
Provision for Income Taxes		1,815			17,683
Net Income		26,634			83,330
Net Income Attributable to Stepan Company		$ 26,643			$ 83,353
Net Income Per Diluted Common Share Attributable to Stepan Company		$ 1.14			$ 3.58
Balance Sheet [Abstract]
Inventories		$ 174,940			$ 174,940
Other Non-Current Assets		12,400			12,400
Total Assets		1,491,422			1,491,422
Deferred Income Taxes		19,667			19,667
Retained Earnings		885,215			885,215
Total Liabilities and Equity		1,491,422			1,491,422
Statement of Cash Flows [Abstract]
Net Income		$ 26,634			83,330
Deferred Income Taxes					(1,553)
Changes in assets and liabilities:
Inventories					$ 22,879

[1] 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 the last in, first out (LIFO) basis to the first in, first out (FIFO) basis.

[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 retained earnings and net income amounts for the noted line items have been 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 2019 amounts for the noted line items include an adjustment related to the Company’s first quarter 2019 adoption of ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income.