Divestitures Divestitures (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended					3 Months Ended
	Sep. 30, 2013 acre	Sep. 30, 2013 Southern Alberta Basin [Member]	Jun. 30, 2013 Barnett Shale [Member]	Sep. 30, 2013 Barnett Shale [Member]	Apr. 30, 2013 Barnett Shale [Member]	Sep. 30, 2013 Tokyo Gas [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Approximate Gross Acres Eni West Texas Total	52,500
Eni Funding of West Texas Transaction	$ 52.0
Joint Interest Ownership After Complete Three Phases	50.00%
Phase One Eni Funding Percent	100.00%
Completion of Phase One Joint Interest Earned	50.00%
Phase Two Approximate Gross Acres	7,500
Eni Funding Percent for Phase Three	100.00%
Equity Method Investment, Ownership Percentage					25.00%
Significant Acquisitions and Disposals, Terms		46	485
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds		42.3			464.0
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds, Estimated Final Adjustment					0.6
Significant Acquisitions and Disposals, Gain (Loss) on Sale or Disposal, Pretax				7.8	341.1
Oil and Gas Property Decrease due to Gain						$ 110.7