Annual report pursuant to Section 13 and 15(d)

Cost Incurred in Oil and Gas Property Acquisition Exploration and Development Activities (Detail)

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Cost Incurred in Oil and Gas Property Acquisition Exploration and Development Activities (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Costs incurred:      
Proved property acquisitions $ 239.8 [1] $ 369.9 [1] $ 277.3 [1]
Exploration 151.3 [1],[2],[3] 92.7 [1],[2],[3] 70.8 [1],[2],[3]
Development 363.7 [1] 203.7 [1] 158.3 [1]
Unproved property acquisitions 26.5 [1],[4] 95.1 [1],[4] 19.7 [1],[4]
Total costs incurred in oil and gas property acquisition, exploration and development activities $ 781.3 [1] $ 761.4 [1] $ 526.1 [1]
[1] Includes additions (reductions) to our ARO of $86.9 million, $32.8 million and $106.1 million during 2012, 2011 and 2010, respectively, associated with acquisitions, liabilities incurred and revisions of estimates. Refer to Note 5.
[2] Includes seismic costs of $6.2 million, $8.0 million and $5.8 million incurred during 2012, 2011 and 2010, respectively.
[3] Includes geological and geophysical costs charged to expense of $6.2 million, $6.8 million and $4.3 million during 2012, 2011 and 2010, respectively.
[4] The amounts for 2012, 2011 and 2010 include capitalized interest associated with properties classified as unproved at December 31, 2012, 2011 and 2010, respectively.