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Summary of Significant Accounting Policies - Recent Accounting Developments (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cost of operations | $ 2,051 | $ 1,841 | $ 3,609 | $ 3,704
Other income, net | $ (20) | 14 | $ (23) | 26
ASU 2017-07 |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cost of operations |  | 4 |  | 8
Other income, net |  | $ 4 |  | $ 8

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