Inventories - Components of Inventories (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
Inventory
Total inventories	$ 253.2	$ 278.6	$ 216.1
Non-utility LPG and natural gas
Inventory
Total inventories	177.0	188.4	135.4
Gas Utility natural gas
Inventory
Total inventories	18.6	39.5	21.8
Materials, supplies and other
Inventory
Total inventories	$ 57.6	$ 50.7	$ 58.9