Marketable Securities - General Information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Marketable securities sold during period, fair value	$ 3.6	$ 127.6
Marketable securities sold during period, amortized cost basis	3.6	127.7
Net realized losses		0.1
Proceeds from sale of marketable securities	3.5	127.0
Reclassification unrealized net holding losses		$ 0.1
Maximum
Schedule of Available-for-sale Securities [Line Items]
Net realized losses	0.1
Reclassification unrealized net holding losses	$ 0.1