Fair Value Measurements - Financial Assets and Financial Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Financial assets at fair value:
Fixed maturities	$ 22,928	$ 23,138
Equity securities	$ 415
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:
Fixed maturities	$ 22,928	23,138
Equity securities	459	588
Short-term investments	316	199
Real estate funds priced at NAV as a practical expedient	239
Other derivatives [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	53	2
Financial liabilities at fair value:
Derivative liabilities	10	25
Federal government and agency [Member]
Financial assets at fair value:
Fixed maturities	710	779
Federal government and agency [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	710	779
State and local government [Member]
Financial assets at fair value:
Fixed maturities	985	1,287
State and local government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	985	1,287
Foreign government [Member]
Financial assets at fair value:
Fixed maturities	2,362	2,487
Foreign government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	2,362	2,487
Corporate [Member]
Financial assets at fair value:
Fixed maturities	18,361	18,088
Corporate [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	18,361	18,088
Mortgage and other asset-backed [Member]
Financial assets at fair value:
Fixed maturities	510	497
Mortgage and other asset-backed [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	510	497
Private equity securities [Member]
Financial assets at fair value:
Equity securities		70
Fair Value Inputs Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	209	253
Equity securities	384	412
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:
Fixed maturities	209	253
Fair Value Inputs Level 1 [Member] \| State and local government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 1 [Member] \| Foreign government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 1 [Member] \| Corporate [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 1 [Member] \| Mortgage and other asset-backed [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	22,341	22,256
Equity securities	43	73
Short-term investments	316	199
Fair Value Inputs Level 2 [Member] \| Other derivatives [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Derivative assets	53	2
Financial liabilities at fair value:
Derivative liabilities	10	25
Fair Value Inputs Level 2 [Member] \| Federal government and agency [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	501	526
Fair Value Inputs Level 2 [Member] \| State and local government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	985	1,287
Fair Value Inputs Level 2 [Member] \| Foreign government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	2,356	2,442
Fair Value Inputs Level 2 [Member] \| Corporate [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	18,127	17,658
Fair Value Inputs Level 2 [Member] \| Mortgage and other asset-backed [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	372	343
Fair Value Inputs Level 3 [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	378	629
Equity securities	32	103
Subtotal	378	629
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] \| Federal government and agency [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 3 [Member] \| State and local government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	0	0
Fair Value Inputs Level 3 [Member] \| Foreign government [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	6	45
Fair Value Inputs Level 3 [Member] \| Corporate [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	234	430
Fair Value Inputs Level 3 [Member] \| Mortgage and other asset-backed [Member] \| Fair Value, Measurements, Recurring [Member]
Financial assets at fair value:
Fixed maturities	$ 138	$ 154

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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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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