Supplemental Oil and Gas Disclosures - Cost Incurred in Oil and Gas Property Acquisition Exploration and Development Activities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Costs incurred:
Proved properties acquisitions	$ 111.5	[1]	$ 96.9	[1]	$ 239.8	[1]
Exploration	411.1	[1],[2],[3]	215.3	[1],[2],[3]	151.3	[1],[2],[3]
Development	198.7	[1]	352.9	[1]	363.7	[1]
Unproved property acquisitions	3.1	[1],[4]	26.3	[1],[4]	26.5	[1],[4]
Total costs incurred in oil and gas property acquisition, exploration and development activities	$ 724.4	[1]	$ 691.4	[1]	$ 781.3	[1]

[1] Includes net additions from capitalized ARO of $88.0 million, $50.6 million and $86.9 million during 2014, 2013 and 2012, respectively, associated with acquisitions, liabilities incurred and revisions of estimates.

[2] Includes seismic costs of $9.0 million, $8.9 million and $6.2 million incurred during 2014, 2013 and 2012, respectively.

[3] Includes geological and geophysical costs charged to expense of $7.3 million, $5.9 million and $6.2 million during 2014, 2013 and 2012, respectively.

[4] The amounts for unproved property acquisitions include capitalized interest associated with unproved properties acquired during the period.