Discontinued Operations (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended			9 Months Ended
	Jan. 03, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of tax		$ 43.2	$ (2.3)		$ 42.2	$ 1,871.5
Liability associated with EFW		64.0			64.0
Current assets of discontinued operations		0.0			0.0		$ 10.2
Current liabilities of discontinued operations		0.0			0.0		15.7
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		43.2	$ (2.3)		42.2	1,871.5
Adjustment to prior gain (loss) on disposal, before-tax		13.6
Tax adjustments, settlements, and unusual provisions		29.6
Gain (Loss) on sale (disposal), after-tax						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
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						1,928.8
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
Gain on sale, after-tax, per share	$ 8.34
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)	(57.3)
Gain (Loss) on sale (disposal), after-tax						$ (47.1)
Loss on disposal, before-tax				$ 59.3
Loss on disposal, after tax				$ 47.1
Current assets of discontinued operations							$ 10.2