Assets and Liabilities, at Fair Value - Details of Level 3 Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Assets and Liabilities, at Fair Value [Abstract]
Total Level 3 Assets	$ 4.3	$ 4.2
Level 3 assets for which the company bears economic exposure	4.3	4.2
Assets	3,039.7	2,848.0	2,953.0
Assets, Fair Value Disclosure	$ 1,469.0	$ 1,627.7
Total level 3 assets as a percentage of total financial assets at fair value	0.30%	0.30%
Level 3 assets for which the company bears economic exposure as a percertange of total assets	0.10%	0.10%
Total level 3 assets as a percentage of total assets	0.10%	0.10%