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Taxation (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income Tax Examination [Line Items] |  |  |  |
Income tax (benefit) credit related to 2017 Tax Act |  | $ (25) | $ (450) | $ 0
Valuation allowance | $ 99 | 79 | 99 |
Unrecognized tax benefits that would affect the effective tax rate | 13 | 14 | 13 |
Liabilities recorded for tax-related interest and penalties | 3 | 3 | $ 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% | 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 |  | $ 491 |  |
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 |  | $ 262 |  |
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% |  |
U.K. |  |  |  |
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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