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Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Equity |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Investment committee equity target allocation percentage of pension assets | 70.00% |
Fixed Income |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Investment committee equity target allocation percentage of pension assets | 30.00% |
Carrying Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Amortized cost basis of investments | $ 340 | $ 91

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