Income Taxes - Effective Tax Rate Reconciliation and Pretax Income (Loss) and Revenue by Tax Jurisdiction (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ 77	$ 38
Tax-exempt interest	(14)	(14)
Change in liability for uncertain tax positions	1	1
Other	1	2
Total provision (benefit) for income taxes	65us-gaap_IncomeTaxExpenseBenefit	27us-gaap_IncomeTaxExpenseBenefit
Effective tax rate (as a percent)	24.20%	38.80%
Income (loss) before provision for income taxes	266	69
Revenue	369us-gaap_Revenues	195us-gaap_Revenues
United States [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes	223	113
Revenue	300	195
Bermuda [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes	50	(37)
Revenue	73	1
United Kingdom [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes	(7)	(7)
Revenue	$ (4)	$ (1)

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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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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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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from 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 increase (decrease) in income tax contingencies. Including, but not limited to, domestic tax contingency, foreign tax contingency, state and local tax contingency, and other contingencies.

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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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