Investments - Investments - Securities Lending, Repurchase Agreements, and Other Collateral Transactions (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Securities Lending Transactions:
Gross amount of securities on loan	$ 165	$ 606
Gross amount of associated liability for collateral received	169	621
Repurchase agreements:
Securities for Reverse Repurchase Agreements	14	15
Securities Loaned, Collateral, Obligation to Return Securities	0	34
Assets Held by Insurance Regulators	2,500	2,300
US Treasury Securities [Member]
Repurchase agreements:
Debt Securities, Available-for-sale, Restricted	$ 34	37
Minimum
Repurchase agreements:
Securities Sold Under Agreements to Repurchase, Percentage of Fair Value Required to be Paid in Cash at Sale	95.00%
Fair Value Measured at Net Asset Value Per Share [Member]
Repurchase agreements:
Deposit Assets	$ 48	38
Syndicate Policyholders [Member] \| Debt Securities [Member]
Repurchase agreements:
Debt Securities, Available-for-sale, Restricted	604	447
Syndicate Policyholders [Member] \| Short-term investments
Repurchase agreements:
Restricted Investments, Current	24	$ 189
Lloyd's of London [Member] \| Debt Securities [Member]
Repurchase agreements:
Debt Securities, Available-for-sale, Restricted	$ 64

Securities Loaned, Collateral, Obligation to Return

No definition available.

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- Definition

Securities Loaned, Collateral, Obligation to Return Securities

+ References

No definition available.

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- Definition

Securities Sold Under Agreements to Repurchase, Percentage of Fair Value Required to be Paid in Cash at Sale

+ References

No definition available.

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- Definition

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=75763970&loc=SL75763979-209797

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- Definition

Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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- Definition

The carrying amount of the asset transferred to a third party to serve as a deposit, which typically serves as security against failure by the transferor to perform under terms of an agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8,17)
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- Definition

This element represents the current portion of investments which are not defined as or included in marketable (debt, equity, or other) securities that are pledged or subject to withdrawal restrictions.

+ References

No definition available.

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- Definition

The carrying value of securities purchased to be resold in reverse repurchase transactions as of the balance sheet date.

+ References

No definition available.

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- References

No definition available.

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- Definition

Amount, before effects of master netting arrangements, of securities loaned to entities in exchange for collateral. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

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- References

No definition available.

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