Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Effective Tax Rate Reconciliation
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ 165	$ 51	$ 241	$ 43
Tax-exempt interest	(14)	(15)	(43)	(46)
Change in liability for uncertain tax positions	4	0	(3)	1
Other	(3)	1	(1)	3
Total provision (benefit) for income taxes	152	37	194	1
Effective tax rate (as a percent)	28.20%	20.60%	29.70%	3.70%
Income (loss) before provision for income taxes	536	179	653	37
Revenue	670	338	959	731
United States
Pretax Income and Revenue by Tax Jurisdiction
Federal statutory income tax rate (as a percent)	35.00%
Effective Tax Rate Reconciliation
Income (loss) before provision for income taxes	469	145	688	123
Revenue	548	284	846	675
Bermuda
Effective Tax Rate Reconciliation
Income (loss) before provision for income taxes	67	34	(35)	(86)
Revenue	122	54	113	56
UK
Pretax Income and Revenue by Tax Jurisdiction
Federal statutory income tax rate (as a percent)			23.00%
Value added tax rate			20.00%
Effective Tax Rate Reconciliation
Effective tax rate (as a percent)	23.25%
Income (loss) before provision for income taxes	0	0	0	0
Revenue	$ 0	$ 0	$ 0	$ 0
UK \| April 1, 2014 to April 1, 2015 [Member]
Pretax Income and Revenue by Tax Jurisdiction
Federal statutory income tax rate (as a percent)			21.00%
UK \| Subsequent to April 1, 2015 [Member]
Pretax Income and Revenue by Tax Jurisdiction
Federal statutory income tax rate (as a percent)			20.00%
UK \| Subsequent to April 1, 2013
Pretax Income and Revenue by Tax Jurisdiction
Federal statutory income tax rate (as a percent)	23.00%
UK \| April 1, 2012 to April 1, 2013 [Member]
Pretax Income and Revenue by Tax Jurisdiction
Federal statutory income tax rate (as a percent)	24.00%
UK \| Prior to April 1, 2012
Pretax Income and Revenue by Tax Jurisdiction
Federal statutory income tax rate (as a percent)	26.00%
Effective Tax Rate Reconciliation
Effective tax rate (as a percent)	24.50%

Effective Income Tax Rate Reconciliation, Value Added Tax Rate, Percent

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The amount of income tax expense or benefit for the period computed by applying the statutory tax rates to pretax income from continuing operations in each jurisdiction.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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The net amount of all increases and decreases in unrecognized tax benefits for the period.

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