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Summary of Significant Accounting Policies - Other Balance Sheet Information (Details) - USD ($) $ in Millions | Sep. 30, 2019 | Dec. 31, 2018
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Accounting Policies [Abstract] |  |
Accounts receivable allowance for doubtful accounts | $ 57 | $ 32
Property, plant and equipment accumulated depreciation | 1,759 | 1,811
Intangible assets accumulated amortization | 1,215 | 1,149
Out-of-market contracts accumulated amortization | $ 0 | $ 37

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