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Inventory (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Inventory Disclosure [Abstract] |  |  |
Fuel oil | $ 37 | $ 73 |
Coal | 73 | 93 |
Natural gas | 22 | 21 |
Spare parts | 195 | 196 |
Total Inventory | 327 | 383 |
Inventory [Line Items] |  |  |
Inventory lower of cost or market adjustment | 29 | $ 0 | $ 2
Fuel Oil |  |  |
Inventory [Line Items] |  |  |
Inventory lower of cost or market adjustment | $ 29 |  |

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- Definition Carrying amount as of the balance sheet date of coal, which is a combustible rock or minerals which are extracted from the ground and used as fuel. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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