Fair Value Measurements - Financial Assets and Financial Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Financial assets at fair value:
Debt securities	$ 23,861	$ 22,928
Financial liabilities at fair value:
Redemption Frequency	quarterly redemption frequency
Unfunded Commitments	$ 57
Minimum [Member]
Financial liabilities at fair value:
Redemption Notice Period	45 days
Maximum [Member]
Financial liabilities at fair value:
Redemption Notice Period	90 days
Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	$ 23,861	22,928
Equity securities	91	459
Short-term investments	257	316
Real estate funds priced at NAV as a practical expedient	217	239
Other derivatives [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	63	53
Financial liabilities at fair value:
Derivative liabilities	14	10
Federal government and agency [Member]
Financial assets at fair value:
Debt securities	767	710
Federal government and agency [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	767	710
State and local government [Member]
Financial assets at fair value:
Debt securities	898	985
State and local government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	898	985
Foreign government [Member]
Financial assets at fair value:
Debt securities	2,239	2,362
Foreign government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	2,239	2,362
Corporate [Member]
Financial assets at fair value:
Debt securities	19,430	18,361
Corporate [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	19,430	18,361
Mortgage and other asset-backed [Member]
Financial assets at fair value:
Debt securities	527	510
Mortgage and other asset-backed [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	527	510
Fair Value Inputs Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	219	209
Equity securities	8	384
Short-term investments	0	0
Fair Value Inputs Level 1 [Member] \| Other derivatives [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	0	0
Financial liabilities at fair value:
Derivative liabilities	0	0
Fair Value Inputs Level 1 [Member] \| Federal government and agency [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	219	209
Fair Value Inputs Level 1 [Member] \| State and local government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	0	0
Fair Value Inputs Level 1 [Member] \| Foreign government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	0	0
Fair Value Inputs Level 1 [Member] \| Corporate [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	0	0
Fair Value Inputs Level 1 [Member] \| Mortgage and other asset-backed [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	0	0
Fair Value Inputs Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	23,243	22,341
Equity securities	51	43
Short-term investments	257	316
Fair Value Inputs Level 2 [Member] \| Other derivatives [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	63	53
Financial liabilities at fair value:
Derivative liabilities	14	10
Fair Value Inputs Level 2 [Member] \| Federal government and agency [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	548	501
Fair Value Inputs Level 2 [Member] \| State and local government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	898	985
Fair Value Inputs Level 2 [Member] \| Foreign government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	2,211	2,356
Fair Value Inputs Level 2 [Member] \| Corporate [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	19,199	18,127
Fair Value Inputs Level 2 [Member] \| Mortgage and other asset-backed [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	387	372
Fair Value Inputs Level 3 [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	399	378
Equity securities	32	32
Short-term investments	0	0
Fair Value Inputs Level 3 [Member] \| Other derivatives [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	0	0
Financial liabilities at fair value:
Derivative liabilities	0	0
Fair Value Inputs Level 3 [Member] \| Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Subtotal	399	378
Fair Value Inputs Level 3 [Member] \| Federal government and agency [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	0	0
Fair Value Inputs Level 3 [Member] \| State and local government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	0	0
Fair Value Inputs Level 3 [Member] \| Foreign government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	28	6
Fair Value Inputs Level 3 [Member] \| Corporate [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	231	234
Fair Value Inputs Level 3 [Member] \| Mortgage and other asset-backed [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Debt securities	$ 140	$ 138

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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Amount of 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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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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States the frequency (for example, annually, semi-annually, quarterly) with which the entity can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments), by major category.

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Notice period the entity is required to deliver before it can redeem an investment calculated using net asset value per share, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, investments calculated by per unit, per membership interest, other equity or ownership interest and alternative investments.

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The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

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Amount of short-term investments classified as other.

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