Assets and Liabilities, at Fair Value - Details of Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Assets and Liabilities, at Fair Value [Abstract]
Total Level 3 Assets	$ 3.2	$ 3.7
Level 3 assets for which the company bears economic exposure	3.2	3.7
Assets	5,951.3	$ 5,070.0	$ 3,039.7
Assets, Fair Value Disclosure	$ 3,156.5
Total level 3 assets as a percentage of total assets	0.10%	0.10%
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 financial assets at fair value	0.10%	0.10%