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Supplemental Oil and Gas Reserve Information - Unaudited - Changes in Standardized Measure of Discounted Future Net Cash Flows Applicable to Proved Oil and Natural Gas Reserves (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Standardized Measure of Discounted Future Net Cash Flows |  |  |
January 1 | $ 2,483,065 | $ 1,914,330 | $ 3,981,661
Net changes in prices and production costs | 881,993 | (367,527) | (3,201,195)
Net changes in future development costs | (60,929) | 69,992 | 150,333
Sales of oil and natural gas, net | (769,625) | (403,739) | (477,755)
Purchases of reserves in place | 6,518 | 533,505 | 14,378
Sales of reserves in place | (9,024) | (57,770) | 0
Revisions of previous quantity estimates | (78,942) | 333,398 | (946,729)
Previously estimated development costs incurred | 157,386 | 91,518 | 216,981
Accretion of discount | 262,776 | 202,272 | 548,141
Net change in income taxes | (238,354) | (36,303) | 1,391,358
Changes in timing and other | 4,364 | 37,463 | (172,681)
December 31 | 3,300,695 | 2,483,065 | 1,914,330
Extensions |  |  |
Standardized Measure of Discounted Future Net Cash Flows |  |  |
Proved oil and gas reserves increase | 661,467 | 165,926 | 409,838
Discoveries |  |  |
Standardized Measure of Discounted Future Net Cash Flows |  |  |
Proved oil and gas reserves increase | $ 0 | $ 0 | $ 0

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- Definition Amount of decrease in standardized measure of discounted future net cash flow as a result of sales of minerals in place. + 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 (e) -URI http://asc.fasb.org/extlink&oid=68064819&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 extensions, discoveries and improved recovery of 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 (d) -URI http://asc.fasb.org/extlink&oid=68064819&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=68064819&loc=d3e62500-109447 + Details
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