Derivative Instruments and Fair Value Measurement (Details 2) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Sep. 30, 2013	Mar. 31, 2013	Sep. 30, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, carrying amount	$ 1,239.8	$ 1,200.9	$ 967.1	$ 903.9
Cash and cash equivalents, fair value	1,239.8	1,200.9
Short-term investments, carrying amount	485.8	372.7
Short-term investments, fair value	485.8	372.7
Short-term debt, carrying amount	342.5	179.0
Short-term debt, fair value	342.5	179.0
Long-term debt, carrying amount	905.4	905.1
Long-term debt, fair value	1,095.8	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	1,112.4	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	127.4	121.9
Short-term investments, fair value	485.8	372.7
Short-term debt, fair value	342.5	179.0
Long-term debt, fair value	1,095.8	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