Marketable Securities - General Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Marketable securities sold during period, fair value	$ 3.6	$ 0.0	$ 3.6	$ 127.6
Marketable securities sold during period, amortized cost basis	3.6	$ 0.0	3.6	127.7
Net realized losses				0.2
Proceeds from sale of marketable securities			3.5	127.0
Reclassification unrealized net holding losses				$ 0.2
Unrealized losses on available for sale securities presented as a component of accumulated other comprehensive income	0.0		0.0
Maximum
Schedule of Available-for-sale Securities [Line Items]
Net realized losses	$ 0.1		0.1
Reclassification unrealized net holding losses			$ 0.1