Income Taxes - Effective Tax Rate Reconciliation and Pretax Income (Loss) and Revenue by Tax Jurisdiction (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ 14	$ 44	$ 49	$ 129
Tax-exempt interest	(6)	(12)	(12)	(24)
Bargain purchase gain	0	0	0	(20)
Change in liability for uncertain tax positions	1	(37)	(6)	(35)
State taxes	(1)	1	1	6
Foreign taxes	6	2	3	3
Taxes on reinsurance	(1)	(1)	(1)	(2)
Other	(1)	0	(2)	(5)
Total provision (benefit) for income taxes	$ 12	$ (3)	$ 32	$ 52
Effective tax rate (as a percent)	13.20%	(1.90%)	10.40%	10.00%
Income (loss) before income taxes	$ 87	$ 150	$ 304	$ 522
Revenue	221	308	514	835
United States [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	71	130	246	376
Revenue	190	265	437	739
Bermuda [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	24	28	73	161
Revenue	36	47	88	104
United Kingdom [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	(8)	(8)	(15)	(15)
Revenue	$ (5)	$ (4)	$ (11)	$ (8)

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

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Effective Income Tax Rate Reconciliation, Taxes on Reinsurance, 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 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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