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
		Mar. 31, 2019	Mar. 31, 2018		Dec. 31, 2018
Income Statement [Abstract]
Cost of Sales	[1]	$ 404,561	$ 408,137
Gross Profit	[1]	84,609	91,198
Operating Income	[1],[2]	29,738	41,283
Income Before Provision for Income Taxes	[1],[2]	31,030	39,292
Provision for Income Taxes	[1]	6,052	7,347
Net Income	[1],[2]	24,978	31,945	[3]
Net Income Attributable to Stepan Company	[1],[2]	$ 24,984	$ 31,952
Net Income Per Diluted Common Share Attributable to Stepan Company	[1],[2]	$ 1.07	$ 1.37
Balance Sheet [Abstract]
Inventories		$ 215,028			$ 231,528
Other Non-Current Assets	[2]	10,332			9,549
Total Assets	[2]	1,526,000			1,514,614
Deferred Income Taxes	[2]	24,158			24,961
Retained Earnings	[2],[4]	861,773			837,107
Total Liabilities and Equity	[2]	1,526,000			1,514,614
Statement of Cash Flows [Abstract]
Net Income	[1],[2]	24,978	$ 31,945	[3]
Deferred Income Taxes	[2]	(1,857)	756
Changes in assets and liabilities:
Inventories	[2]	16,231	(10,621)
As Originally Reported Under LIFO [Member]
Income Statement [Abstract]
Cost of Sales			409,765
Gross Profit			89,570
Operating Income			39,655
Income Before Provision for Income Taxes			37,664
Provision for Income Taxes			6,948
Net Income			30,716
Net Income Attributable to Stepan Company			$ 30,723
Net Income Per Diluted Common Share Attributable to Stepan Company			$ 1.31
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			$ 30,716
Deferred Income Taxes			357
Changes in assets and liabilities:
Inventories			(8,993)
Effect of Change [Member]
Income Statement [Abstract]
Cost of Sales			(1,628)
Gross Profit			1,628
Operating Income			1,628
Income Before Provision for Income Taxes			1,628
Provision for Income Taxes			399
Net Income		(1,132)	1,229
Net Income Attributable to Stepan Company			$ 1,229
Net Income Per Diluted Common Share Attributable to Stepan Company			$ 0.06
Balance Sheet [Abstract]
Inventories		(32,863)			31,363
Other Non-Current Assets		1,483			(1,415)
Total Assets		(31,380)			29,948
Deferred Income Taxes		(6,589)			6,289
Retained Earnings		(24,791)			23,659
Total Liabilities and Equity		(31,380)			$ 29,948
Statement of Cash Flows [Abstract]
Net Income		(1,132)	$ 1,229
Deferred Income Taxes		(368)	399
Changes in assets and liabilities:
Inventories		$ 1,500	$ (1,628)

[1] The 2018 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 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 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.