Assets and Liabilities, at Fair Value - Details of Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2015	Sep. 30, 2014
Assets and Liabilities, at Fair Value [Abstract]
Total Level 3 Assets	$ 4.0	$ 4.3
Level 3 assets for which the company bears economic exposure	4.0	4.3
Assets	4,486.8	3,039.7
Assets, Fair Value Disclosure	$ 2,824.3	$ 1,469.0
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%	0.30%
Suriwongse Debt Investment Balance	$ 3.5