Inventories (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Inventory, Net [Abstract]
Total inventories	$ 61,861	$ 67,334	$ 97,451
Gas Utility Natural Gas [Member]
Inventory, Net [Abstract]
Total inventories	51,838	57,663	87,735
Materials, Supplies and Other [Member]
Inventory, Net [Abstract]
Total inventories	$ 10,023	$ 9,671	$ 9,716