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Schedule II - Valuation and Qualifying Accounts | 12 Months Ended
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Dec. 31, 2019
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract] |
Schedule II - Valuation and Qualifying Accounts | Schedule II Valuation and Qualifying Accounts For the Years Ended December 31, 2019, 2018 and 2017
(In millions) Description | Balance at Beginning of Period |  | Additions Charged to Costs and Expenses |  | Deductions |  | Other |  | Balance at End of Period
Allowance for doubtful accounts: |  |  |  |  |  |  |  |  |
2019 | $ | 8 |  |  | $ | 17 |  |  | $ | 16 |  |  | $ | — |  |  | $ | 9 |
2018 | 7 |  |  | 17 |  |  | 16 |  |  | — |  |  | 8 |
2017 | 10 |  |  | 20 |  |  | 23 |  |  | — |  |  | 7 |
Obsolescence allowance—spare parts: |  |  |  |  |  |  |  |  |
2019 | $ | 412 |  |  | $ | 76 |  |  | $ | 63 |  |  | $ | — |  |  | $ | 425 |
2018 | 354 |  |  | 73 |  |  | 15 |  |  | — |  |  | 412 |
2017 | 295 |  |  | 75 |  |  | 17 |  |  | 1 |  |  | 354 |
Valuation allowance for deferred tax assets: |  |  |  |  |  |  |  |  |
2019 | $ | 59 |  |  | $ | — |  |  | $ | 1 |  |  | $ | — |  |  | $ | 58 |
2018 | 63 |  |  | 2 |  |  | 6 |  |  | — |  |  | 59 |
2017 | 68 |  |  | 11 |  |  | 27 |  |  | 11 |  |  | 63 |

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- Definition The entire disclosure for valuation and qualifying accounts and reserves. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 09 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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- References No definition available. + Details
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