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							6 Months Ended
		Jun. 30, 2019		Mar. 31, 2019	Jun. 30, 2018		Mar. 31, 2018	[3]	Jun. 30, 2019		Jun. 30, 2018		Dec. 31, 2018
Income Statement [Abstract]
Cost of Sales	[1]	$ 380,044			$ 429,885				$ 784,605		$ 838,022
Gross Profit	[1]	92,959			89,981				177,568		181,179
Operating Income	[1],[2]	41,065			45,386				70,803		86,669
Income Before Provision for Income Taxes	[1],[2]	39,534			43,198				70,564		82,490
Provision for Income Taxes	[1]	9,324			9,746				15,376		17,093
Net Income		30,210	[1],[2]	$ 24,978	33,452	[1],[2],[3]	$ 31,945		55,188	[1],[2]	65,397	[1],[2]
Net Income Attributable to Stepan Company	[1],[2]	$ 30,218			$ 33,454				$ 55,202		$ 65,406
Net Income Per Diluted Common Share Attributable to Stepan Company	[1],[2]	$ 1.30			$ 1.44				$ 2.37		$ 2.80
Balance Sheet [Abstract]
Inventories	[2]	$ 216,278							$ 216,278				$ 231,528
Other Non-Current Assets	[2]	12,098							12,098				9,549
Total Assets	[2]	1,528,968							1,528,968				1,514,614
Deferred Income Taxes	[2]	28,271							28,271				24,961
Retained Earnings	[2],[4]	886,349							886,349				837,107
Total Liabilities and Equity	[2]	1,528,968							1,528,968				1,514,614
Statement of Cash Flows [Abstract]
Net Income		30,210	[1],[2]	$ 24,978	$ 33,452	[1],[2],[3]	$ 31,945		55,188	[1],[2]	$ 65,397	[1],[2]
Deferred Income Taxes	[2]								223		3,034
Changes in assets and liabilities:
Inventories	[2]								15,515		(16,738)
Effect of Change [Member]
Income Statement [Abstract]
Cost of Sales		(3,500)			(701)				(2,000)		(2,329)
Gross Profit		3,500			701				2,000		2,329
Operating Income		3,500			701				2,000		2,329
Income Before Provision for Income Taxes		3,500			701				2,000		2,329
Provision for Income Taxes		860			172				492		571
Net Income		2,640			529				1,508		1,758
Net Income Attributable to Stepan Company		$ 2,640			$ 529				$ 1,508		$ 1,758
Net Income Per Diluted Common Share Attributable to Stepan Company		$ 0.11			$ 0.03				$ 0.06		$ 0.07
Balance Sheet [Abstract]
Inventories		$ (29,363)							$ (29,363)				31,363
Other Non-Current Assets		1,415							1,415				(1,415)
Total Assets		(27,948)							(27,948)				29,948
Deferred Income Taxes		(5,797)							(5,797)				6,289
Retained Earnings		(22,151)							(22,151)				23,659
Total Liabilities and Equity		(27,948)							(27,948)				$ 29,948
Statement of Cash Flows [Abstract]
Net Income		2,640			$ 529				1,508		$ 1,758
Deferred Income Taxes									492		571
Changes in assets and liabilities:
Inventories									(2,000)		$ (2,329)
As Computed under LIFO [Member]
Income Statement [Abstract]
Cost of Sales		376,544							782,605
Gross Profit		96,459							179,568
Operating Income		44,565							72,803
Income Before Provision for Income Taxes		43,034							72,564
Provision for Income Taxes		10,184							15,868
Net Income		32,850							56,696
Net Income Attributable to Stepan Company		$ 32,858							$ 56,710
Net Income Per Diluted Common Share Attributable to Stepan Company		$ 1.41							$ 2.43
Balance Sheet [Abstract]
Inventories		$ 186,915							$ 186,915
Other Non-Current Assets		13,513							13,513
Total Assets		1,501,020							1,501,020
Deferred Income Taxes		22,474							22,474
Retained Earnings		864,198							864,198
Total Liabilities and Equity		1,501,020							1,501,020
Statement of Cash Flows [Abstract]
Net Income		$ 32,850							56,696
Deferred Income Taxes									715
Changes in assets and liabilities:
Inventories									$ 13,515

[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.