Discontinued Operations (Narrative) (Details) $ / shares in Units, $ in Millions			3 Months Ended	12 Months Ended
	Jan. 03, 2017 USD ($)	Oct. 01, 2016	Dec. 31, 2016 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($) $ / shares		Sep. 30, 2016 USD ($)		Sep. 30, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of tax				$ 42.2	$ 1,866.0		$ (460.5)
Current assets of discontinued operations				0.0	10.2
Current liabilities of discontinued operations				0.0	15.7
Liabilities associated with EfW				63.3	65.3
Deferred tax assets, valuation allowance				105.0	107.7
Unrecognized tax benefits				233.6	146.4		90.2		$ 83.8
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of tax				42.2	1,866.0		(460.5)
Tax benefit related to settlement and release of reserves				25.6
Adjustment to prior gain (loss) on disposal, net of tax				17.6
Assets of discontinued operations				0.0
Current liabilities of discontinued operations					15.7
Gain on sale, after tax					1,780.5	[1]	(846.6)
Electronic Materials Division (EMD) | Discontinued Operations, Spinoff
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of tax							240.2
Distribution ratio of common stock in spin-off		0.5
Gain on sale, 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]
Income (loss) from discontinued operations, net of tax					1,923.9		183.5
Proceeds from sale	$ 3,800.0
Gain on sale, before tax					2,870.0
Gain on sale, after tax					$ 1,827.6	[1]	0.0
Gain on sale, after tax, per share | $ / shares					$ 8.32
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]
Income (loss) from discontinued operations, net of tax				$ (1.0)	$ (57.9)		(884.2)	[2]
Current assets of discontinued operations					10.2
Gain on sale, after tax					$ (47.1)		(846.6)
Loss on disposal, before tax			$ 59.3				945.7
Loss on disposal, after tax			$ 47.1				846.6
Deferred tax assets, valuation allowance							58.0
Unrecognized tax benefits							$ 7.9

[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.

[2] The loss from operations of discontinued operations for EfW primarily relates to project suspension costs, land lease obligations, and administrative costs.