Reconciliations of Equity - Reconciliations of Total Equity (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reconciliations of total equity
Beginning Balance				$ 740,914	$ 635,916
Net income		$ 32,923	$ 27,896	63,639	59,808
Dividends				(10,160)	(9,225)
Common stock purchases	[1]			(15,184)	(3,191)
Stock option exercises				3,274	2,518
Defined benefit pension adjustments, net of tax		752	567	1,505	1,132
Translation adjustments		(35,059)	5,997	(26,242)	16,451
Derivative instrument activity, net of tax		(2)	(3)	(5)	(5)
Other	[2]			4,293	5,295
Ending Balance		762,034	708,699	762,034	708,699
Stepan Company Equity [Member]
Reconciliations of total equity
Beginning Balance				740,096	634,604
Net income				63,648	59,795
Dividends				(10,160)	(9,225)
Common stock purchases	[1]			(15,184)	(3,191)
Stock option exercises				3,274	2,518
Defined benefit pension adjustments, net of tax				1,505	1,132
Translation adjustments				(26,227)	16,418
Derivative instrument activity, net of tax				(5)	(5)
Other	[2]			4,293	5,295
Ending Balance		761,240	707,341	761,240	707,341
Noncontrolling Interests' Equity [Member]
Reconciliations of total equity
Beginning Balance	[3]			818	1,312
Net income	[3]			(9)	13
Translation adjustments	[3]			(15)	33
Ending Balance	[3]	$ 794	$ 1,358	$ 794	$ 1,358

[1] Includes the value of Company shares purchased in the open market and from the Company’s retirement plans and the value of Company common shares 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.