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Fair Value Measurements (Narrative) (Detail) - Commodity Swaps And Options [Member] - Level 3 [Member] | Dec. 31, 2020 Grade
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
Weighted Average Risk Grade | 14.0
Weighted average loss severity in the event of default on commodity swaps | 10.00%

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