Income Taxes - Effective Tax Rate Reconciliation and Pretax Income (Loss) and Revenue by Tax Jurisdiction (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ 116	$ 150	$ 245	$ 234
Tax-exempt interest	(12)	(12)	(36)	(38)
Gain on bargain purchase	0	(125)	(20)	(125)
State taxes	1	0	7	1
Change in liability for uncertain tax positions	8	8	(27)	10
Effect of provision to tax return filing adjustments	(8)	(16)	(8)	(16)
Other	0	(4)	(4)	(4)
Total provision (benefit) for income taxes	$ 105	$ 1	$ 157	$ 62
Effective tax rate (as a percent)	33.60%	0.30%	18.80%	8.30%
Income (loss) before income taxes	$ 313	$ 480	$ 835	$ 746
Revenue	623	566	1,458	1,207
United States [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	337	432	713	681
Revenue	566	499	1,305	1,041
Bermuda [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	(18)	56	143	88
Revenue	61	69	165	170
United Kingdom [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	(6)	(8)	(21)	(23)
Revenue	$ (4)	$ (2)	$ (12)	$ (4)

Effective Income Tax Rate Reconciliation, Business Combination, Gain On Bargain Purchase, Amount

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Line items represent pretax income taxes and revenue financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

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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 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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