Business Restructuring and Cost Reduction Plans (Narrative) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended												3 Months Ended	12 Months Ended
	Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2012 Employees		Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2012 Contract Actions [Member]	Sep. 30, 2012 Cost Reduction Plan [Member]	Sep. 30, 2009 Cost Reduction Plan [Member]	Sep. 30, 2012 Cost Reduction Plan [Member] Contract Actions [Member]	Sep. 30, 2012 Photovoltaic Market Actions [Member]		Sep. 30, 2012 Photovoltaic Market Actions [Member] Contract Actions [Member]	Sep. 30, 2012 Photovoltaic Market Actions [Member] Contract Actions [Member]		Sep. 30, 2012 Polyurethane Intermediates Business Actions [Member]		Sep. 30, 2012 Polyurethane Intermediates Business Actions [Member] Contract Actions [Member]
Restructuring and Related Cost [Line Items]
Business restructuring and cost reduction plans	$ 240.6	[1]	$ 0	[1]	$ 86.8	[1]	$ 0	[1]	$ 327.4	[2]	$ 0	[2]	$ 0	[2]	$ 100.0	$ 86.8	$ 298.2	$ 0	$ 186.0	[3]	$ 93.5	$ 93.5	[3]	$ 54.6	[4]	$ 6.5	[4]
Cost reduction plan charge, after-tax									222.4							60.6	200.3		127.0					34.8
Cost reduction plan charge, per share									$ 1.03							$ 0.28	$ 0.94		$ 0.59					$ 0.16
Severance and other costs																80.8
Write-down of certain assets																6.0
Proceeds from sale of assets and rights to contract									32.7
Expense for asset write offs net of proceeds									$ 26.6
Number of positions to be eliminated									600

[1]   For additional information, see Note 4, Business Restructuring and Cost Reduction Plans.

[2] Information about how this charge related to the businesses at the segment level is discussed in Note 4, Business Restructuring and Cost Reduction Plans.

[3]   Other includes the write-down of inventory to its net realizable value, the write-down of accounts receivable, and expected losses on purchase commitments.

[4]   Charge is net of $32.7 in proceeds received in cash at closing for certain PUI assets and the rights to a supply contract.