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Fair Value Measurements Narrative (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Impairment | $ 0 | $ 8 |  |
Holding Company |  |  |  |
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Impairment | 0 | 0 |  |
Recurring measurement | Level 3 |  |  |  |
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value of assets measured at fair value on a recurring basis | 302 | 214 | $ 279 | $ 212
Recurring measurement | Holding Company | Trading securities |  |  |  |
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value of assets measured at fair value on a recurring basis | $ 297 |  | $ 274 |
Nonrecurring measurement |  |  |  |
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Impairment of PP&E |  | 3 |  |
Inventory Write-down |  | $ 5 |  |

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- Definition Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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