Investments - Investments - Securities Lending, Repurchase Agreements, and Other Collateral Transactions (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Securities Lending Transactions:
Gross amount of securities on loan	$ 603	$ 820
Gross amount of associated liability for collateral received	618	840
Repurchase agreements:
Gross amount of recognized liabilities for repurchase agreements	0	72
Gross amount of collateral pledged related to repurchase agreements	0	73
Securities for Reverse Repurchase Agreements	52	64
Securities Loaned, Collateral, Obligation to Return Securities	0	3
Assets Held by Insurance Regulators	2,400	2,200
Short-term investments	2,927	4,283
Debt Securities [Member]
Repurchase agreements:
Debt Securities, Available-for-sale, Restricted	548
Short-term Investments [Member]
Repurchase agreements:
Short-term investments	0
US Treasury Securities [Member]
Repurchase agreements:
Debt Securities, Available-for-sale, Restricted	$ 37	47
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	$ 55	$ 0

Securities Loaned, Collateral, Obligation to Return

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Securities Loaned, Collateral, Obligation to Return Securities

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Securities Sold Under Agreements to Repurchase, Collateral

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

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

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

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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.

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Amount of short-term investments classified as other.

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

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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.

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Amount, before effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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