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Rollforward of Construction-In-Process (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Property, Plant and Equipment [Line Items]      
Ending Balance $ 396,141,000 $ 370,551,000  
Projects under exploration and development
     
Property, Plant and Equipment [Line Items]      
Beginning Balance 78,653,000 54,697,000 33,617,000
Cost incurred during the year 4,202,000 23,956,000 39,374,000
Write off of unsuccessful exploration costs (2,639,000)   (3,050,000)
Reclassification of exploration and development projects to drilling and construction (12,651,000)   (15,244,000)
Ending Balance 67,565,000 78,653,000 54,697,000
Projects under exploration and development | Up-front bonus lease costs
     
Property, Plant and Equipment [Line Items]      
Beginning Balance 36,832,000 33,600,000 15,867,000
Cost incurred during the year   3,232,000 17,733,000
Write off of unsuccessful exploration costs (1,160,000)    
Reclassification of exploration and development projects to drilling and construction (1,687,000)    
Ending Balance 33,985,000 36,832,000 33,600,000
Projects under exploration and development | Exploration and development costs
     
Property, Plant and Equipment [Line Items]      
Beginning Balance 40,223,000 20,997,000 17,698,000
Cost incurred during the year 3,782,000 19,226,000 21,483,000
Write off of unsuccessful exploration costs (1,479,000)   (2,940,000)
Reclassification of exploration and development projects to drilling and construction (10,224,000)   (15,244,000)
Ending Balance 32,302,000 40,223,000 20,997,000
Projects under exploration and development | Interest capitalized
     
Property, Plant and Equipment [Line Items]      
Beginning Balance 1,598,000 100,000 52,000
Cost incurred during the year 420,000 1,498,000 158,000
Write off of unsuccessful exploration costs     (110,000)
Reclassification of exploration and development projects to drilling and construction (740,000)    
Ending Balance 1,278,000 1,598,000 100,000
Projects under construction
     
Property, Plant and Equipment [Line Items]      
Beginning Balance 291,898,000 215,937,000 484,978,000
Cost incurred during the year 228,435,000 252,273,000 286,407,000
Reclassification of exploration and development projects to drilling and construction 12,651,000   15,244,000
Reclassification of completed projects to property, plant and equipment (204,408,000) (176,312,000) (570,692,000)
Ending Balance 328,576,000 291,898,000 215,937,000
Projects under construction | Up-front bonus lease costs
     
Property, Plant and Equipment [Line Items]      
Beginning Balance 31,179,000   3,179,000
Cost incurred during the year     28,000,000
Reclassification of exploration and development projects to drilling and construction 1,687,000    
Reclassification of completed projects to property, plant and equipment (3,706,000)    
Ending Balance 29,160,000   31,179,000
Projects under construction | Drilling and construction costs
     
Property, Plant and Equipment [Line Items]      
Beginning Balance 246,878,000 176,968,000 442,218,000
Cost incurred during the year 216,894,000 242,066,000 249,072,000
Reclassification of exploration and development projects to drilling and construction 10,224,000   15,244,000
Reclassification of completed projects to property, plant and equipment (190,123,000) (172,156,000) (529,566,000)
Ending Balance 283,873,000 246,878,000 176,968,000
Projects under construction | Interest capitalized
     
Property, Plant and Equipment [Line Items]      
Beginning Balance 13,841,000 7,790,000 39,581,000
Cost incurred during the year 11,541,000 10,207,000 9,335,000
Reclassification of exploration and development projects to drilling and construction 740,000    
Reclassification of completed projects to property, plant and equipment (10,579,000) (4,156,000) (41,126,000)
Ending Balance $ 15,543,000 $ 13,841,000 $ 7,790,000