Related Party Transactions (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2012 Energy Services [Member]	Sep. 30, 2011 Energy Services [Member]	Sep. 30, 2010 Energy Services [Member]	Sep. 30, 2012 SCAAs [Member] Energy Services [Member] Storage_Agreement	Sep. 30, 2011 SCAAs [Member] Energy Services [Member]	Sep. 30, 2010 SCAAs [Member] Energy Services [Member]	Sep. 30, 2012 Exclusive of transactions pursuant SCAAs [Member] Energy Services [Member]	Sep. 30, 2011 Exclusive of transactions pursuant SCAAs [Member] Energy Services [Member]	Sep. 30, 2010 Exclusive of transactions pursuant SCAAs [Member] Energy Services [Member]	Sep. 30, 2012 Other current liabilities [Member] SCAAs [Member] Energy Services [Member]	Sep. 30, 2011 Other current liabilities [Member] SCAAs [Member] Energy Services [Member]	Sep. 30, 2012 Inventories [Member] Energy Services [Member] ft3	Sep. 30, 2011 Inventories [Member] Energy Services [Member] ft3	Nov. 01, 2012 New Storage Agreement [Member] SCAAs [Member] Energy Services [Member] Storage_Agreement
Related Party Transaction [Line Items]
Number of storage agreements							2										2
Storage agreement, term (in years)							3 years										3 years
Related party costs incurred	$ 9,326	$ 12,125	$ 14,209				$ 24,344	$ 35,231	$ 21,826	$ 30,752	$ 30,093	$ 25,941
Related party security deposits													15,000	15,000
Volume of gas storage inventory (in bcf of natural gas)															7,600,000,000	7,500,000,000
Natural gas storage inventory, related parties, current															21,217	35,686
Revenue from related parties				65,705	85,655	62,074
Purchases from related party				$ 53,435	$ 53,617	$ 31,157