Change in Method of Accounting for Inventory Valuation - Summary of Prior Year Financial Statement Line Items that have Been Affected by Retrospective Change in Accounting Principle (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)
As Originally Reported Under LIFO [Member]
Income Statement [Abstract]
Cost of Sales					430,586						840,351
Gross Profit					89,280						178,850
Operating Income					44,685						84,340
Income Before Provision for Income Taxes					42,497						80,161
Provision for Income Taxes					9,574						16,522
Net Income					32,923						63,639
Net Income Attributable to Stepan Company					$ 32,925						$ 63,648
Net Income Per Diluted Common Share Attributable to Stepan Company					$ 1.41						$ 2.73
Balance Sheet [Abstract]
Inventories													200,165
Other Non-Current Assets													10,964
Total Assets													1,484,666
Deferred Income Taxes													18,672
Retained Earnings													813,448
Total Liabilities and Equity													1,484,666
Statement of Cash Flows [Abstract]
Net Income					$ 32,923						$ 63,639
Deferred Income Taxes											2,463
Changes in assets and liabilities:
Inventories											(14,409)
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)

[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.