Discontinued Operations (Narrative) (Details) $ / shares in Units, $ in Millions			3 Months Ended		12 Months Ended
	Jan. 03, 2017 USD ($) $ / shares	Oct. 01, 2016	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2017 USD ($)		Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred tax assets, valuation allowance					$ 107.7		$ 165.1
Unrecognized tax benefits					146.4		90.2	$ 83.8	$ 93.1
Electronic Materials Division (EMD) | Discontinued Operations, Spinoff
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Distribution ratio of common stock in spin-off		0.5
Loss on disposal, after tax							0.0
Performance Materials Division (PMD) | Discontinued Operations, Held-for-sale or Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale	$ 3,800.0
Gain on sale, before tax	2,870.0
Gain on sale, after tax	$ 1,828.0				1,827.6	[1]
Gain on sale, after tax, per share | $ / shares	$ 8.32
Loss on disposal, after tax							0.0
Energy-from-Waste (EfW) | Discontinued Operations, Held-for-sale or Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale, after tax					(47.1)
Loss on disposal, before tax			$ 59.3	$ 945.7	59.3		945.7
Loss on disposal, after tax			47.1	846.6			846.6
Deferred tax assets, valuation allowance				58.0
Unrecognized tax benefits				$ 7.9
Energy-from-Waste (EfW) | Discontinued Operations, Held-for-sale or Disposed of by Sale | Asset Actions
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on disposal, before tax			6.3		6.3		913.5
Energy-from-Waste (EfW) | Discontinued Operations, Held-for-sale or Disposed of by Sale | Contract Actions/ Other
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on disposal, before tax			$ 53.0		$ 53.0		$ 32.2

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