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Taxation (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Income Tax Examination [Line Items] |  |  |  |
Unrecognized Tax Benefits | $ 13 | $ 47 | $ 14 | $ 13
Income Tax Credits and Adjustments |  | 19 | 0 |
Income tax (benefit) credit related to 2017 Tax Act |  | 0 | (25) | $ (450)
Valuation allowance |  | 114 | 79 |
Unrecognized tax benefits that would affect the effective tax rate |  | 28 | 14 |
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense |  | 5 |  |
Liabilities recorded for tax-related interest and penalties |  | 8 | 3 |
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount |  |  | (475) |
Impact on deferred tax liability [Member] |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount |  |  | (743) |
Impact on Deferred Tax Asset [Member] |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount |  |  | 250 |
Other impact [Member] |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount |  |  | $ 18 |
Tax Year 2017 [Member] |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Applicable income tax rates |  |  |  | 35.00%
Tax Year 2018 [Member] |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Applicable income tax rates |  |  | 21.00% |
Domestic Tax Authority [Member] |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Net operating loss carry-forwards |  | $ 496 |  |
Switzerland |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Applicable income tax rates |  | 7.83% |  |
Foreign Tax Authority [Member] |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Foreign tax credit carry-forward |  | $ 247 |  |
Bermuda |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Applicable income tax rates |  | 0.00% |  |
UNITED STATES |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Applicable income tax rates |  | 21.00% |  |
UNITED KINGDOM |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Applicable income tax rates |  | 19.00% |  |
Scenario, Adjustment [Member] |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount |  |  | $ (25) |
Scenario, Plan [Member] |  |  |  |
Income Tax Examination [Line Items] |  |  |  |
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount | $ (450) |  |  |

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