Assets and Liabilities, at Fair Value - Available for Sale Securities in OCI (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Assets and Liabilities, at Fair Value [Abstract]
Equity Investments in Exchange Stock - Cost	$ 3.9	$ 3.7
Equity Investments in Exchange Stock - Fair Value	8.3	6.4
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	0.0	0.0	$ 5.4
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	$ 0.0	$ 0.0	$ 2.7