Goodwill and Other Intangible Assets Other Intangible Assets (Details) - USD ($) $ in Millions		3 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		$ 10,750			$ 4,152
Finite-Lived Intangible Assets, Accumulated Amortization		(1,207)			(820)
Finite-Lived Intangible Assets, Net		9,543			3,332
Indefinite-lived Intangible Assets (Excluding Goodwill)		1,881			8,723
Intangible Assets, Gross (Excluding Goodwill)		12,631			12,875
Total intangible assets - net		11,424			12,055
In Process Research and Development [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets (Excluding Goodwill)	[1]	10			576
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)		90	$ 54	$ 85
Impairment of Intangible Assets Indefinite lived Excluding Goodwill After Tax		69	$ 41	$ 66
Germplasm [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets (Excluding Goodwill)	[2]				6,265
Trademarks and Trade Names [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets (Excluding Goodwill)		1,871			1,871
Other Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets (Excluding Goodwill)		0			11
Germplasm [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	[2]	6,265
Finite-Lived Intangible Assets, Accumulated Amortization	[2]	(63)
Finite-Lived Intangible Assets, Net	[2]	6,202
Customer-Related Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		1,977			1,985
Finite-Lived Intangible Assets, Accumulated Amortization		(268)			(154)
Finite-Lived Intangible Assets, Net		1,709			1,831
Developed Technology Rights [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	[1]	1,463			974
Finite-Lived Intangible Assets, Accumulated Amortization	[1]	(370)			(163)
Finite-Lived Intangible Assets, Net	[1]	1,093			811
Trademarks and Trade Names [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		166			180
Finite-Lived Intangible Assets, Accumulated Amortization		(86)			(92)
Finite-Lived Intangible Assets, Net		80			88
Favorable Supply Contract [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		475			475
Finite-Lived Intangible Assets, Accumulated Amortization		(207)			(111)
Finite-Lived Intangible Assets, Net		268			364
Other Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	[3]	404			538
Finite-Lived Intangible Assets, Accumulated Amortization	[3]	(213)			(300)
Finite-Lived Intangible Assets, Net	[3]	$ 191			$ 238

[1] During the first quarter of 2019, the company announced the launch of its Qrome^® corn hybrids following the receipt of regulatory approval from China. As a result, the company reclassified the amounts from indefinite-lived IPR&D to developed technology.

[2] Beginning on October 1, 2019, the company changed its indefinite life assertion of the germplasm assets to definite lived with a useful life of 25 years.  This change is the result of a more focused development effort of new seed products coupled with an intent to out license select germplasm on a non-exclusive basis. Prior to changing the useful life of the germplasm assets, the company tested the assets for impairment under ASC 350 - Intangibles, Goodwill and Other, concluding the assets were not impaired.

[3] Primarily consists of sales and farmer networks, marketing and manufacturing alliances and noncompetition agreements.