Inventories - Components of Inventories (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Inventory
Total inventories	$ 307.3	$ 278.6	$ 228.2
Non-utility LPG and natural gas
Inventory
Total inventories	216.4	188.4	150.9
Gas Utility natural gas
Inventory
Total inventories	34.6	39.5	25.8
Materials, supplies and other
Inventory
Total inventories	$ 56.3	$ 50.7	$ 51.5