Business Restructuring and Cost Reduction Plans (Narrative) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended			6 Months Ended			12 Months Ended		3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012		Mar. 31, 2013	Mar. 31, 2012		Sep. 30, 2012	Sep. 30, 2012 Contract Actions [Member]	Mar. 31, 2012 Cost Reduction Plan [Member] Employees	Mar. 31, 2012 Cost Reduction Plan [Member] Merchant Gases [Member]	Mar. 31, 2012 Cost Reduction Plan [Member] Tonnage Gases [Member]	Mar. 31, 2012 Cost Reduction Plan [Member] Electronics and Performance Materials [Member]	Sep. 30, 2012 Photovoltaic Market 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	$ 0	$ 86.8	[1]	$ 0	$ 86.8	[1]	$ 327.4	$ 100.0	$ 86.8	$ 77.3	$ 3.8	$ 5.7	$ 186.0	$ 93.5	$ 54.6	[2]	$ 6.5
Business restructuring and cost reduction plans, after-tax							222.4		60.6				127.0		34.8
Business restructuring and cost reduction plans, per share							$ 1.03		$ 0.28				$ 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] Information about how this charge related to the businesses at the segment level is discussed in Note 4, Business Restructuring and Cost Reduction Plans.

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