Income Taxes - Effective Tax Rate Reconciliation and Pretax Income (Loss) and Revenue by Tax Jurisdiction (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ 85	$ 18
Tax-exempt interest	(12)	(13)
Gain on bargain purchase	(20)	0
State taxes	5	1
Other	(3)	0
Total provision (benefit) for income taxes	$ 55	$ 6
Effective tax rate (as a percent)	14.70%	10.00%
Income (loss) before income taxes	$ 372	$ 65
Revenue	527	245
United States [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	246	55
Revenue	474	205
Bermuda [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	133	17
Revenue	57	42
United Kingdom [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	(7)	(7)
Revenue	$ (4)	$ (2)

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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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 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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