Taxation (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Examination [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	$ 18	$ 1
Valuation allowance	56	57
Unrecognized tax benefits that would affect the effective tax rate	8
Tax-related interest and penalties reported in the consolidated statements of operations	(8)	3	(1)
Liabilities recorded for tax-related interest and penalties	12	22
Domestic Tax Authority [Member]
Income Tax Examination [Line Items]
Net operating loss carry-forwards	157
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	$ 76
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	24.50%