Debt (Narrative) (Details)		1 Months Ended			3 Months Ended								12 Months Ended
	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 CLF	Sep. 30, 2016 USD ($)		Jun. 30, 2016 USD ($)	[1]	Mar. 31, 2016 USD ($)	[1]	Dec. 31, 2015 USD ($)	[1]	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)		Sep. 30, 2015 USD ($)	Mar. 30, 2017 USD ($)
Debt Disclosure [Abstract]
Short-term debt, weighted average interest rate		1.10%			1.10%								4.60%	1.10%
Cash paid for interest, net of amounts capitalized													$ 125,900,000	$ 120,600,000		$ 96,800,000
Debt Instrument [Line Items]
Loss on extinguishment of debt					$ 6,900,000	[1]	$ 0		$ 0		$ 0		0	6,900,000	[1]	16,600,000
Payments on long-term debt													483,900,000	480,400,000		$ 699,400,000
Versum Financing
Debt Instrument [Line Items]
Loss on extinguishment of debt		$ 6,900,000												6,900,000
Extinguishment of debt amount														$ 418,300,000
Foreign Line of Credit | Foreign Subsidiary Facility
Debt Instrument [Line Items]
Credit facility, amount borrowed and outstanding													23,400,000
Revolving Credit Facility | 2017 Credit Agreement
Debt Instrument [Line Items]
Credit facility, amount borrowed and outstanding													$ 0
Debt instrument, term	5 years
Maximum borrowing capacity	$ 2,500,000,000.0
Revolving Credit Facility | 2017 Credit Agreement | Maximum
Debt Instrument [Line Items]
Covenant, ratio of total debt to total capitalization	70.00%
Revolving Credit Facility | 2013 Credit Agreement
Debt Instrument [Line Items]
Credit facility, amount borrowed and outstanding																	$ 0
Maximum borrowing capacity																	$ 2,690,000,000.0
Early termination penalties	$ 0
Long-term Debt | CLP Series E bonds 6.3%
Debt Instrument [Line Items]
Loss on extinguishment of debt			$ 16,600,000
Payments on long-term debt			146,600,000
Extinguishment of debt amount			$ 130,000,000	CLF 3,000,000
Interest rate, stated percentage			6.30%	6.30%												6.30%

[1] For additional information, see Note 15, Debt.