Taxation (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014
Income Tax Examination [Line Items]
Valuation allowance	$ 17	$ 38	$ 17
Unrecognized tax benefits that would not affect the effective tax rate		17
Unrecognized tax benefits that would affect the effective tax rate	6	16	6
Tax-related interest and penalties reported in the consolidated statements of operations		1	(1)
Liabilities recorded for tax-related interest and penalties	9	4	$ 9
Income Tax Reconciliation, Change To Deferred Taxes Related to Unrealized Foreign Exchange Net	$ 89
Domestic Tax Authority [Member]
Income Tax Examination [Line Items]
Net operating loss carry-forwards		$ 325
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		$ 142
Bermuda
Income Tax Examination [Line Items]
Applicable income tax rates		0.00%
U.S.
Income Tax Examination [Line Items]
Applicable income tax rates		35.00%
U.K.
Income Tax Examination [Line Items]
Applicable income tax rates		20.25%