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Derivative Financial Instruments (Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions) (Detail) bbl in Thousands, MMBTU in Thousands, $ in Thousands | Dec. 31, 2017 USD ($) MMBTU bbl | Dec. 31, 2016 USD ($) MMBTU bbl
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Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member] |  |
Derivative Instruments [Line Items] |  |
Oil/Natural Gas Derivative Assets Notional Amount | bbl | 253 | 227
Oil/Natural Gas Derivative Assets, Estimated Fair Value | $ 193 | $ 206
Oil/Natural Gas Derivative Liabilities Notional Amount | bbl | 2,731 | 944
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value | $ (13,448) | $ (4,400)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member] |  |
Derivative Instruments [Line Items] |  |
Oil/Natural Gas Derivative Assets Notional Amount | MMBTU | 5,927 | 0
Oil/Natural Gas Derivative Assets, Estimated Fair Value | $ 1,399 | $ 0
Oil/Natural Gas Derivative Liabilities Notional Amount | MMBTU | 3,917 | 1,299
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value | $ (326) | $ (1,357)
Customer Counterparties [Member] | Oil Commodity Derivative [Member] |  |
Derivative Instruments [Line Items] |  |
Oil/Natural Gas Derivative Assets Notional Amount | bbl | 2,731 | 944
Oil/Natural Gas Derivative Assets, Estimated Fair Value | $ 13,709 | $ 4,580
Oil/Natural Gas Derivative Liabilities Notional Amount | bbl | 253 | 227
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value | $ (187) | $ (206)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member] |  |
Derivative Instruments [Line Items] |  |
Oil/Natural Gas Derivative Assets Notional Amount | MMBTU | 3,917 | 1,299
Oil/Natural Gas Derivative Assets, Estimated Fair Value | $ 340 | $ 1,393
Oil/Natural Gas Derivative Liabilities Notional Amount | MMBTU | 5,927 | 0
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value | $ (1,366) | $ 0

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