Inventories - Components of Inventories (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Inventory
Total inventories	$ 222.0	$ 318.2	$ 228.3
Non-utility LPG and natural gas
Inventory
Total inventories	159.7	231.7	163.0
Gas Utility natural gas
Inventory
Total inventories	3.4	37.3	3.5
Materials, supplies and other
Inventory
Total inventories	$ 58.9	$ 49.2	$ 61.8