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Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract] |
Public Utilities, Inventory | Inventory is recorded at average cost and consisted of the following:
(Millions of Dollars) |  | Dec. 31, 2018 |  | Dec. 31, 2017
Inventories |  |  |  |
Materials and supplies |  | $ | 271 |  |  | $ | 311 |
Fuel |  | 170 |  |  | 186 |
Natural gas |  | 107 |  |  | 113 |
|  | $ | 548 |  |  | $ | 610 |

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- Definition Tabular disclosure of all information related to inventories for utilities. + References No definition available. + Details
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