Assets and Liabilities, at Fair Value - Details of Level 3 Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Sep. 30, 2014
Assets and Liabilities, at Fair Value [Abstract]
Total Level 3 Assets	$ 4.1	$ 4.3
Level 3 assets for which the company bears economic exposure	4.1	4.3
Assets	4,462.6	3,039.7
Assets, Fair Value Disclosure	2,708.8	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	0.20%	0.30%
Suriwongse Debt Investment Balance	$ 3.6