Inventories - Components of Inventories (Details) - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Inventory
Total inventories	$ 228.2	$ 210.3	$ 246.8
Non-utility LPG and natural gas
Inventory
Total inventories	150.9	129.8	148.6
Gas Utility natural gas
Inventory
Total inventories	25.8	29.2	35.9
Materials, supplies and other
Inventory
Total inventories	$ 51.5	$ 51.3	$ 62.3