Reserve for Unpaid Losses and Loss Adjustment Expenses - Discounted Reserves, Group (Details) - Group Insurance Policies - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted Average Discount Rate [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	$ 9,071	$ 6,382	$ 6,565
Less: amount of discount	1,536	1,303	1,382
Carrying value of liability for unpaid losses and loss adjustment expenses	$ 7,535	$ 5,079	$ 5,183
Weighted average discount rate	3.50%	4.30%	4.40%
Minimum
Weighted Average Discount Rate [Line Items]
Range of discount rates	2.10%	3.00%	3.00%
Maximum
Weighted Average Discount Rate [Line Items]
Range of discount rates	8.00%	8.00%	8.00%

Range of Discount Rates

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- Definition

Short-Duration Contracts, Undiscounted Liabilities, Amount

+ References

No definition available.

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- Definition

The carrying amount as of the balance sheet date of loss reserves carried at present value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65670685&loc=d3e14881-158438

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 5.N.Q1)
-URI http://asc.fasb.org/extlink&oid=27015749&loc=d3e569934-122904

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- Definition

Amount of cumulative discount related to the time value of money deducted from short-duration insurance contract liabilities that are reported at present value.

+ References

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- Definition

Line items represent 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.

+ References

No definition available.

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- Definition

The average discount rate applied to insurance contracts to determine future policy benefits to be paid to or on behalf of policyholders.

+ References

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