Discontinued Operations (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended			6 Months Ended
		Jan. 03, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Liability associated with EFW			$ 68.0			$ 68.0
Current assets of discontinued operations			0.0			0.0		$ 10.2
Current liabilities of discontinued operations			0.0			0.0		15.7
Income (Loss) From Discontinued Operations, net of tax			0.0	$ 1,825.6		(1.0)	$ 1,873.8
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (Loss) on sale (disposal), after-tax				1,832.5			1,785.4
Current liabilities of discontinued operations								15.7
Assets of discontinued operations			0.0			0.0
Liabilities of discontinued operations			0.0			0.0
Income (Loss) From Discontinued Operations, net of tax			$ 0.0	1,825.6		$ (1.0)	1,873.8
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 (Loss) on sale (disposal), after-tax		$ 1,833.0		1,832.5			1,832.5
Gain on sale, after-tax, per share		$ 8.34
Income (Loss) From Discontinued Operations, net of tax				1,828.1			1,928.8
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 (Loss) on sale (disposal), after-tax	[1]			0.0			(47.1)
Loss on disposal, before-tax					$ 59.3
Loss on disposal, after tax					$ 47.1
Current assets of discontinued operations								$ 10.2
Income (Loss) From Discontinued Operations, net of tax	[1]			$ (2.5)			$ (55.0)

[1] The loss from operations of discontinued operations for EfW primarily relates to land lease obligations, administrative costs, and costs incurred for project exit activities.