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)