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Schedule II - Valuation and Qualifying Accounts | 12 Months Ended
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Dec. 31, 2018
Valuation and Qualifying Accounts [Abstract] |
Schedule II - Valuation and Qualifying Accounts | Schedule II Valuation and Qualifying Accounts For the Years Ended December 31, 2018, 2017 and 2016
(In millions) Description | Balance at Beginning of Period |  | Additions Charged to Costs and Expenses |  | Deductions (a) |  | Other |  | Balance at End of Period
Allowance for doubtful accounts: |  |  |  |  |  |  |  |  |
2018 | $ | 7 |  |  | $ | 17 |  |  | $ | 16 |  |  | $ | — |  |  | $ | 8 |
2017 | 10 |  |  | 20 |  |  | 23 |  |  | — |  |  | 7 |
2016 | 18 |  |  | 18 |  |  | 26 |  |  | — |  |  | 10 |
Obsolescence allowance—spare parts: |  |  |  |  |  |  |  |  |
2018 | $ | 354 |  |  | $ | 73 |  |  | $ | 15 |  |  | $ | — |  |  | $ | 412 |
2017 | 295 |  |  | 75 |  |  | 17 |  |  | 1 |  |  | 354 |
2016 | 235 |  |  | 61 |  |  | 16 |  |  | 15 |  |  | 295 |
Valuation allowance for deferred tax assets: |  |  |  |  |  |  |  |  |
2018 | $ | 63 |  |  | $ | 2 |  |  | $ | 6 |  |  | $ | — |  |  | $ | 59 |
2017 | 68 |  |  | 11 |  |  | 27 |  |  | 11 |  |  | 63 |
2016 | 48 |  |  | 47 |  |  | 27 |  |  | — |  |  | 68 |
(a) Deduction from reserve for purpose for which reserve was created.

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- Definition The entire disclosure for any allowance and reserve accounts (their beginning and ending balances, as well as a reconciliation by type of activity during the period). Alternatively, disclosure of the required information may be within the footnotes to the financial statements or a supplemental schedule to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -URI http://asc.fasb.org/topic&trid=2122369 + Details
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- References No definition available. + Details
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