Inventories - Components of Inventories (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017
Inventory
Total inventories	$ 228.3	$ 278.6	$ 203.0
Non-utility LPG and natural gas
Inventory
Total inventories	163.0	188.4	143.1
Gas Utility natural gas
Inventory
Total inventories	3.5	39.5	2.4
Materials, supplies and other
Inventory
Total inventories	$ 61.8	$ 50.7	$ 57.5