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)