Other Required Disclosures - Components of Properties (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Sep. 30, 2012
Property, Plant and Equipment [Line Items]
Oil and Gas Properties Unproved	$ 52.9
Oil and natural gas properties (full accounting method)	572.5
Less: Accumulated depletion	5.7
Total oil and natural gas properties, net	619.7
Total properties, at cost	590.6	437.7
Less accumulated depreciation	244.7	216.1
Properties, net	345.9	221.6
Property, Plant and Equipment including Oil and Gas Property, Net	965.6	221.6
Land, buildings and improvements [Member]
Property, Plant and Equipment [Line Items]
Total properties, at cost	121.2	93.6
Gas Gathering and Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Total properties, at cost	21.5
Machinery, equipment and other [Member]
Property, Plant and Equipment [Line Items]
Total properties, at cost	413.9	325.7
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Total properties, at cost	$ 34.0	$ 18.4