Assets and Liabilities, at Fair Value Assets and Liabilities, at Fair Value - Details of Level 3 Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012
Assets and Liabilities, at Fair Value [Abstract]
Total Level 3 Assets	$ 4.5	$ 4.5
Level 3 assets for which the company bears economic exposure	4.5	4.5
Assets	2,939.6	2,958.9
Assets, Fair Value Disclosure	$ 1,634.0	$ 1,764.3
Total level 3 assets as a percentage of total assets	0.20%	0.20%
Level 3 assets for which the company bears economic exposure as a percertange of total assets	0.20%	0.20%
Total level 3 assets as a percentage of total financial assets at fair value	0.30%	0.30%