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Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Derivatives, Fair Value [Line Items] |  |
Derivative assets, Estimated Fair Value | $ 295 | $ 1,569
Derivative liabilities, Estimated Fair Value | (261) | (1,306)
Other Assets [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative assets, Estimated Fair Value | 295 | 1,569
Other Liabilities [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative liabilities, Estimated Fair Value | $ (261) | (1,306)
Oil [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Notional Units | Barrels |
Derivative assets, Notional Amount | $ 33 | 83
Derivative assets, Estimated Fair Value | 279 | 576
Derivative liabilities, Notional Amount | (33) | (83)
Derivative liabilities, Estimated Fair Value | $ (246) | (496)
Natural Gas [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Notional Units | MMBTUs |
Derivative assets, Notional Amount | $ 20 | 1,900
Derivative assets, Estimated Fair Value | 16 | 993
Derivative liabilities, Notional Amount | (20) | (1,900)
Derivative liabilities, Estimated Fair Value | $ (15) | $ (810)

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- Definition Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226006-175313 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 + Details
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