Reconciliations of Equity - Reconciliations of Total Equity (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reconciliations of total equity
Beginning Balance				$ 740,914	$ 635,916
Net income		$ 22,168	$ 21,853	85,807	81,661
Dividends				(15,225)	(13,841)
Common stock purchases	[1]			(16,756)	(6,255)
Stock option exercises				3,488	3,127
Defined benefit pension adjustments, net of tax		788	539	2,293	1,671
Translation adjustments		(2,534)	9,837	(28,776)	26,288
Derivative instrument activity, net of tax		(2)	(2)	(7)	(7)
Other	[2]			6,676	7,711
Ending Balance		778,414	736,271	778,414	736,271
Stepan Company Equity [Member]
Reconciliations of total equity
Beginning Balance				740,096	634,604
Net income				85,816	81,694
Dividends				(15,225)	(13,841)
Common stock purchases	[1]			(16,756)	(6,255)
Stock option exercises				3,488	3,127
Defined benefit pension adjustments, net of tax				2,293	1,671
Translation adjustments				(28,730)	26,229
Derivative instrument activity, net of tax				(7)	(7)
Other	[2]			6,676	7,711
Ending Balance		777,651	734,933	777,651	734,933
Noncontrolling Interests' Equity [Member]
Reconciliations of total equity
Beginning Balance	[3]			818	1,312
Net income	[3]			(9)	(33)
Translation adjustments	[3]			(46)	59
Ending Balance	[3]	$ 763	$ 1,338	$ 763	$ 1,338

[1] Includes the value of Company common stock purchased in the open market and from the Company’s retirement plans and the value of Company common stock tendered by employees to settle statutory withholding taxes related to distributions of deferred performance awards and deferred management incentive compensation and exercises of stock appreciation rights.

[2] Primarily comprised of activity related to stock-based compensation and deferred compensation. Beginning in 2018, also includes beginning retained earnings adjustment as a result of the Company’s first quarter 2018 adoption of ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory.

[3] Reflects the noncontrolling interest in the Company’s China joint venture.