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Supplemental Information on Oil and Natural Gas Producing Activities - Estimate of Changes in Standardized Measure of Discounted Future Net Cash Flows from Proved Reserves (Unaudited) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward] |  |  |
Present value, beginning balance | $ 4,087,752 | $ 4,017,611 | $ 5,840,368
Changes during the year |  |  |
Revenues less production and other costs | (383,293) | (1,043,060) | (1,271,559)
Net changes in prices, production and other costs | (3,813,465) | 331,694 | 271,566
Development costs incurred | 217,596 | 364,262 | 474,275
Net changes in future development costs | 273,437 | (341,183) | (207,729)
Extensions and discoveries | 230,055 | 1,785,963 | 1,406,102
Revisions of previous quantity estimates | (1,354,778) | (77,688) | (296,418)
Accretion of discount | 512,483 | 477,458 | 711,385
Net change in income taxes | 1,426,333 | (256,371) | 477,328
Purchases of reserves in-place | 18,429 | 50,958 | 1,628
Sales of reserves in-place | 0 | (1,058,330) | (3,172,187)
Timing differences and other | 100,013 | (163,562) | (217,148)
Net change for the year | (2,773,190) | 70,141 | (1,822,757)
Present value, ending balance | $ 1,314,562 | $ 4,087,752 | $ 4,017,611

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- Definition Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of passage of time. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 35 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e62500-109447 + Details
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- Definition Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of change in the estimated future income tax on future pretax net cash flows related to the entity's proved oil and gas reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 35 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=51678428&loc=d3e62500-109447 + Details
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