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

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Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, 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 revisions of previously reported income tax expense (benefit).

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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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Income Taxes - Effective Tax Rate Reconciliation and Pretax Income (Loss) and Revenue by Tax Jurisdiction (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit) at statutory rates in taxable jurisdictions									$ 300	$ 316	$ 443
Tax-exempt interest									(49)	(49)	(54)
Goodwill impairment and gain on bargain purchase price									(20)	(125)	(19)
Change in liability for uncertain tax positions									(26)	11	12
Effect of provision to tax return filing adjustments									(8)	(15)	(11)
State taxes									9	3	1
Effect of Tax Act									61	0	0
Other									(6)	(5)	3
Total provision (benefit) for income taxes	$ 104	$ 105	$ (3)	$ 55	$ 74	$ 1	$ 55	$ 6	$ 261	$ 136	$ 375
Effective tax rate (as a percent)									26.30%	13.40%	26.20%
Income (loss) before provision for income taxes	$ 156	$ 313	$ 150	$ 372	$ 271	$ 480	$ 201	$ 65	$ 991	$ 1,017	$ 1,431
Revenue									1,739	1,677	2,207
United States [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes									873	921	1,284
Revenue									1,543	1,442	1,853
Bermuda [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes									145	126	177
Revenue									216	239	361
United Kingdom [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes									(27)	(30)	(30)
Revenue									$ (20)	$ (4)	$ (7)