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Supplemental Information on Oil and Natural Gas Producing Activities - Costs Incurred in Oil and Natural Gas Property Acquisition, Exploration and Development (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Acquisitions of properties      
Proved $ 21,130 $ 1,761,556 $ 58,190
Unproved 100,242 377,185 320,361
Exploration(1) 82,775 [1] 120,438 [1] 98,849 [1]
Development(2) 1,131,269 [2] 1,704,991 [2] 1,296,903 [2]
Total costs incurred $ 1,335,416 $ 3,964,170 $ 1,774,303
[1] Includes seismic costs of $6.7 million, $15.3 million and $4.9 million for 2013, 2012 and 2011, respectively.
[2] Includes loss on the construction of the Century Plant of $50.0 million and $25.0 million for 2012 and 2011, respectively. See Note 11.