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Inventories (Details Textual) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2012
ft3
Sep. 30, 2011
ft3
Jun. 30, 2011
ft3
Inventories (Textual) [Abstract]      
Volume of gas storage inventories released under SCAAs with non-affiliates (in cubic feet) 1,900,000,000 3,900,000,000 2,000,000,000
Carrying value of gas storage inventories released under SCAAs with non-affiliates $ 5.3 $ 19.0 $ 9.6