Derivative Instruments and Fair Value Measurement (Details 2) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, carrying amount	$ 1,191.3	$ 1,200.9	$ 903.9	$ 988.9
Cash and cash equivalents, fair value	1,191.3	1,200.9
Short-term investments, carrying amount	628.5	372.7
Short-term investments, fair value	628.5	372.7
Short-term debt	325.0	179.0
Short-term debt, fair value	325.0	179.0
Long-term debt, carrying amount	905.6	905.1
Long-term debt, fair value	1,119.4	1,072.2
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	1,154.2	1,079.0
Short-term investments, fair value	0	0
Short-term debt, fair value	0	0
Long-term debt, fair value	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	37.1	121.9
Short-term investments, fair value	628.5	372.7
Short-term debt, fair value	325.0	179.0
Long-term debt, fair value	1,119.4	1,072.2
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	0	0
Short-term investments, fair value	0	0
Short-term debt, fair value	0	0
Long-term debt, fair value	$ 0	$ 0