Other Operating Income, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Component of Operating Income [Abstract]
Finance charges	$ 11.8	$ 15.2	$ 12.7
AFUDC associated with pipeline projects	5.5	3.3	0.0
Interest and interest-related income	1.7	0.2	0.8
Utility non-tariff service income	1.5	2.6	4.8
Loss on private equity partnership investment	(11.0)	0.0	0.0
(Losses) gains on sales of fixed assets, net	(3.9)	3.3	11.1
Other, net	4.9	(2.2)	15.0
Total other operating income, net	$ 10.5	$ 22.4	$ 44.4