Restructuring and related activities (2020 Restructuring Program) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Mar. 31, 2020	Mar. 31, 2021
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net	$ 0			$ 96
The Dow Chemical Company [Domain]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net	0			$ 96
2020 Restructuring Program [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net					$ 573
Restructuring Reserve, Settled without Cash		$ (4)	$ (197)
Payments for Restructuring	49	7	1
Restructuring Reserve	$ 315	364	377		$ 315
2020 Restructuring Program [Member] | The Dow Chemical Company [Domain]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			575
Restructuring Reserve, Accrual Adjustment		(2)
restructuring and related cost, percentage	6.00%				6.00%
2020 Restructuring Program [Member] | Accrued and Other Current Liabilities [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	$ 246	227			$ 246
2020 Restructuring Program [Member] | Other Noncurrent Obligations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	69	137			69
2020 Restructuring Program [Member] | Packaging & Specialty Plastics [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			11
2020 Restructuring Program [Member] | Industrial Intermediates & Infrastructure [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			22
2020 Restructuring Program [Member] | Performance Materials & Coatings [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			174
Restructuring Reserve, Accrual Adjustment		3
2020 Restructuring Program [Member] | Corporate Segment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			368
Restructuring Reserve, Accrual Adjustment		(5)
Employee Severance [Member] | 2020 Restructuring Program [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net					297
Restructuring Reserve, Settled without Cash		0	0
Payments for Restructuring	37	7	1
Restructuring Reserve	252	289	296		252
Employee Severance [Member] | 2020 Restructuring Program [Member] | The Dow Chemical Company [Domain]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			297
Restructuring Reserve, Accrual Adjustment		0
Employee Severance [Member] | 2020 Restructuring Program [Member] | Packaging & Specialty Plastics [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			0
Employee Severance [Member] | 2020 Restructuring Program [Member] | Industrial Intermediates & Infrastructure [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			0
Employee Severance [Member] | 2020 Restructuring Program [Member] | Performance Materials & Coatings [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			0
Restructuring Reserve, Accrual Adjustment		0
Employee Severance [Member] | 2020 Restructuring Program [Member] | Corporate Segment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			297
Restructuring Reserve, Accrual Adjustment		0
Asset write-downs and write-offs [Member] | 2020 Restructuring Program [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net					196
Restructuring Reserve, Settled without Cash		(1)	(197)
Payments for Restructuring	0	0	0
Restructuring Reserve	0	0	0		0
Asset write-downs and write-offs [Member] | 2020 Restructuring Program [Member] | The Dow Chemical Company [Domain]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			197
Restructuring Reserve, Accrual Adjustment		(1)
Asset write-downs and write-offs [Member] | 2020 Restructuring Program [Member] | Packaging & Specialty Plastics [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			11
Asset write-downs and write-offs [Member] | 2020 Restructuring Program [Member] | Industrial Intermediates & Infrastructure [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			22
Asset write-downs and write-offs [Member] | 2020 Restructuring Program [Member] | Performance Materials & Coatings [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			117
Restructuring Reserve, Accrual Adjustment		(1)
Asset write-downs and write-offs [Member] | 2020 Restructuring Program [Member] | Corporate Segment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			47
Restructuring Reserve, Accrual Adjustment		0
Costs Associated with Exit and Disposal Activities [Member] | 2020 Restructuring Program [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net					80
Restructuring Reserve, Settled without Cash		(5)	0
Payments for Restructuring	12	0	0
Restructuring Reserve	$ 63	75	81		$ 63
Costs Associated with Exit and Disposal Activities [Member] | 2020 Restructuring Program [Member] | The Dow Chemical Company [Domain]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			81
Restructuring Reserve, Accrual Adjustment		(1)
Costs Associated with Exit and Disposal Activities [Member] | 2020 Restructuring Program [Member] | Packaging & Specialty Plastics [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			0
Costs Associated with Exit and Disposal Activities [Member] | 2020 Restructuring Program [Member] | Industrial Intermediates & Infrastructure [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			0
Costs Associated with Exit and Disposal Activities [Member] | 2020 Restructuring Program [Member] | Performance Materials & Coatings [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			57
Restructuring Reserve, Accrual Adjustment		4
Costs Associated with Exit and Disposal Activities [Member] | 2020 Restructuring Program [Member] | Corporate Segment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and asset related charges - net			$ 24
Restructuring Reserve, Accrual Adjustment		$ (5)