Inventories - Components of Inventories (Details) - USD ($) $ in Millions	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016
Inventory
Total inventories	$ 203.0	$ 210.3	$ 176.5
Non-utility LPG and natural gas
Inventory
Total inventories	143.1	129.8	106.7
Gas Utility natural gas
Inventory
Total inventories	2.4	29.2	3.8
Materials, supplies and other
Inventory
Total inventories	$ 57.5	$ 51.3	$ 66.0