Assets and Liabilities, at Fair Value - Details of Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016
Assets and Liabilities, at Fair Value [Abstract]
Total Level 3 Assets	$ 0.2	$ 3.2
Level 3 assets for which the company bears economic exposure	0.2	3.2
Assets	6,290.7	5,950.3
Assets, Fair Value Disclosure	$ 3,503.7	$ 3,156.5
Total level 3 assets as a percentage of total assets	0.00%	0.10%
Level 3 assets for which the company bears economic exposure as a percertange of total assets	0.00%	0.10%
Total level 3 assets as a percentage of total financial assets at fair value	0.00%	0.10%
Suriwongse Debt Investment Balance		$ 3.0