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Summary of Significant Accounting Policies - New Accounting Pronouncements, Adopted (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018 | Mar. 31, 2018 | Jan. 01, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |
Restricted cash and cash equivalents |  | $ 8 |  |  | $ 17 | $ 6 | $ 78
Cumulative-effect of adopting new accounting guidance |  |  |  | $ 15 |  |  |
Retained Earnings |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |
Cumulative-effect of adopting new accounting guidance |  |  |  | $ 15 |  |  |
ASU 2014-09 | Retained Earnings |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |
Cumulative-effect of adopting new accounting guidance |  |  | $ 16 |  |  |  |
Scenario, Forecast | ASU 2016-02 |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |
Operating leases, right-of-use assets | $ 800 |  |  |  |  |  |
Operating leases, liability | $ 800 |  |  |  |  |  |

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