Assets and Liabilities, at Fair Value - Details of Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017
Assets and Liabilities, at Fair Value [Abstract]
Total Level 3 Assets	$ 0.1	$ 0.1
Level 3 assets for which the company bears economic exposure	0.1	0.1
Assets	6,808.9	6,243.4
Assets, Fair Value Disclosure	$ 2,273.5	$ 2,068.0
Total level 3 assets as a percentage of total assets	0.00%	0.00%
Level 3 assets for which the company bears economic exposure as a percertange of total assets	0.00%	0.00%
Total level 3 assets as a percentage of total financial assets at fair value	0.00%	0.00%