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Summary of Significant Accounting Policies (New Accounting Pronouncements) (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Adoption of New Accounting Standard Current Expected Credit Losses | $ 14 | $ 0 | $ 0
Accounting Standards Update 2016-13 [Member] |  |  |
Adoption of New Accounting Standard Current Expected Credit Losses | 15 |  |
Accounting Standards Update 2018-11 |  |  |
Adoption of New Accounting Standard Current Expected Credit Losses | $ 55 |  |

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