Materials Technologies Separation (Narrative) (Details) $ / shares in Units, $ in Millions				3 Months Ended								12 Months Ended
		Jan. 03, 2017 USD ($)	Oct. 01, 2016	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($) $ / shares
Restructuring Cost and Reserve [Line Items]
Business separation costs, legal and advisory fees, before tax												$ 0.0	$ 0.0	$ 32.5
Pension settlement benefit				$ 0.0	$ 0.0	$ (5.0)	$ 0.0	$ (43.7)	$ 0.0	$ 0.0	$ 0.0	$ (5.0)	$ (43.7)
Materials Technologies
Restructuring Cost and Reserve [Line Items]
Net business separation costs														30.2
Business separation costs, legal and advisory fees, before tax														32.5
Tax benefit related to changes in tax positions on business separation activities														5.5
Materials Technologies | Other Nonoperating Income (Expense)
Restructuring Cost and Reserve [Line Items]
Pension settlement benefit														2.3
Discontinued Operations, Held-for-sale or Disposed of by Sale | Performance Materials Division (PMD)
Restructuring Cost and Reserve [Line Items]
Proceeds from sale		$ 3,800.0
Gain on sale, before tax														2,870.0
Gain on sale, after tax	[1]													$ 1,827.6
Gain on sale, after tax, per share | $ / shares														$ 8.32
Discontinued Operations, Spinoff | Electronic Materials Division (EMD)
Restructuring Cost and Reserve [Line Items]
Distribution ratio of common stock in spin-off			0.5

[1] After-tax gain on sale of $1,827.6 included expense for income tax reserves for uncertain tax positions of $28.0 gross ($21.0 net) in various jurisdictions.