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Fair Value Measurements (Narrative) (Detail) $ in Thousands | Dec. 31, 2017 USD ($) Grade | Dec. 31, 2016 USD ($)
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments using fair value measurement option | $ | $ 0 | $ 0
Commodity Swaps And Options [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Weighted average risk grade of loans underlying swaps measured at fair value using significant unobservable inputs | Grade | 12.0 |
Weighted average loss severity on default of swaps | 15.40% |

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