Inventories - Components of Inventories (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Inventory
Total inventories	$ 293.7	$ 318.2	$ 307.3
Non-utility LPG and natural gas
Inventory
Total inventories	206.7	231.7	216.4
Gas Utility natural gas
Inventory
Total inventories	34.9	37.3	34.6
Materials, supplies and other
Inventory
Total inventories	$ 52.1	$ 49.2	$ 56.3