Marketable Securities - General Information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Marketable securities sold during period, fair value	$ 1.6	$ 3.6
Marketable securities sold during period, amortized cost basis		3.6
Proceeds from sale of marketable securities	1.6	3.5
Reclassification unrealized net holding losses	$ 0.1	0.1
Maximum
Schedule of Available-for-sale Securities [Line Items]
Net realized losses		$ 0.1