Annual report pursuant to Section 13 and 15(d)

Standardized Measure of Discounted Future Net Cash Flows (Detail)

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Standardized Measure of Discounted Future Net Cash Flows (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Standardized Measure of Discounted Future Net Cash Flows      
Future cash inflows $ 6,888,431 $ 7,077,206 $ 3,953,655
Production (1,858,282) (1,862,488) (1,011,552)
Development (655,406) (543,017) (243,570)
Dismantlement and abandonment (508,051) (513,620) (520,490)
Income taxes (1,002,127) (1,126,573) (495,696)
Future net cash inflows before 10% discount 2,864,565 3,031,508 1,682,347
10% annual discount factor (1,018,188) (1,025,131) (503,275)
Standardized measure of discounted future net cash flows $ 1,846,377 $ 2,006,377 $ 1,179,072