Assets and Liabilities, at Fair Value - Details of Level 3 Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Assets and Liabilities, at Fair Value [Abstract]
Assets	$ 2,848.0	$ 2,953.0	$ 2,632.0
Assets, Fair Value Disclosure	$ 1,627.7	$ 1,764.3
Total level 3 assets as a percentage of total assets	0.10%	0.20%
Level 3 assets for which the company bears economic exposure as a percertange of total assets	0.10%	0.20%
Total level 3 assets as a percentage of total financial assets at fair value	0.30%	0.30%