Annual report pursuant to Section 13 and 15(d)

Supplemental Oil and Gas Disclosures - Cost Incurred in Oil and Gas Property Acquisition Exploration and Development Activities (Details)

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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 $ 111.5us-gaap_CostsIncurredAcquisitionOfOilAndGasPropertiesWithProvedReserves [1] $ 96.9us-gaap_CostsIncurredAcquisitionOfOilAndGasPropertiesWithProvedReserves [1] $ 239.8us-gaap_CostsIncurredAcquisitionOfOilAndGasPropertiesWithProvedReserves [1]
Exploration 411.1us-gaap_CostsIncurredExplorationCosts [1],[2],[3] 215.3us-gaap_CostsIncurredExplorationCosts [1],[2],[3] 151.3us-gaap_CostsIncurredExplorationCosts [1],[2],[3]
Development 198.7us-gaap_CostsIncurredDevelopmentCosts [1] 352.9us-gaap_CostsIncurredDevelopmentCosts [1] 363.7us-gaap_CostsIncurredDevelopmentCosts [1]
Unproved property acquisitions 3.1us-gaap_CostsIncurredAcquisitionOfUnprovedOilAndGasProperties [1],[4] 26.3us-gaap_CostsIncurredAcquisitionOfUnprovedOilAndGasProperties [1],[4] 26.5us-gaap_CostsIncurredAcquisitionOfUnprovedOilAndGasProperties [1],[4]
Total costs incurred in oil and gas property acquisition, exploration and development activities $ 724.4us-gaap_CostsIncurredOilAndGasPropertyAcquisitionExplorationAndDevelopmentActivities [1] $ 691.4us-gaap_CostsIncurredOilAndGasPropertyAcquisitionExplorationAndDevelopmentActivities [1] $ 781.3us-gaap_CostsIncurredOilAndGasPropertyAcquisitionExplorationAndDevelopmentActivities [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.