MARKETING AGREEMENT - Net Commission Earned Under the Marketing Agreement (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Marketing Agreement [Line Items]
Contribution expenses	$ (1,778.3)	$ (1,669.5)	$ (1,600.0)
Monsanto Marketing Agreement
Marketing Agreement [Line Items]
Gross commission	80.5	87.7	97.9
Contribution expenses	(18.0)	(18.0)	(18.0)
Amortization of marketing fee	(0.8)	(0.8)	(0.8)
Net commission	61.7	68.9	79.1
Reimbursements associated with Restated Marketing Agreement	54.5	56.1	55.8
Total net sales associated with Restated Marketing Agreement	$ 116.2	$ 125.0	$ 134.9