Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2016	Mar. 31, 2016	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2015
Entity Information [Line Items]
Cash and cash equivalents, Carrying value	$ 1,224.1	$ 1,224.1	$ 993.2	$ 1,303.2	$ 1,383.8
Subsidiaries [Member]
Entity Information [Line Items]
Cash and cash equivalents, Carrying value			$ 39.4	$ 40.5
Other Segments [Member]
Entity Information [Line Items]
General and administrative expense	0.8	2.5
Other income	$ 2.0	$ 3.7