Fair Value Measurements - Financial Assets and Financial Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Financial assets at fair value:
Debt securities	$ 22,573	$ 23,755
Equity securities	126	64
Financial liabilities at fair value
Unfunded commitments	$ 57
Minimum
Financial liabilities at fair value
Redemption Notice Period	45 days
Maximum
Financial liabilities at fair value
Redemption Notice Period	90 days
Recurring
Financial assets at fair value:
Debt securities	$ 22,573	23,755
Equity securities	151	111
Short-term investments	306	423
Real estate funds priced at NAV as a practical expedient	179	184
Recurring | Other Derivatives
Financial assets at fair value:
Derivative assets	239	83
Financial liabilities at fair value
Derivative liabilities	19	18
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt securities	193	197
Equity securities	6	7
Short-term investments	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring | Other Derivatives
Financial assets at fair value:
Derivative assets	0	0
Financial liabilities at fair value
Derivative liabilities	0	0
Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt securities	21,629	23,035
Equity securities	130	72
Short-term investments	306	423
Significant Other Observable Inputs (Level 2) | Recurring | Other Derivatives
Financial assets at fair value:
Derivative assets	239	83
Financial liabilities at fair value
Derivative liabilities	19	18
Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt securities	751	523
Equity securities	15	32
Short-term investments	0	0
Significant Unobservable Inputs (Level 3) | Recurring | Other Derivatives
Financial assets at fair value:
Derivative assets	0	0
Financial liabilities at fair value
Derivative liabilities	0	0
Federal government and agency
Financial assets at fair value:
Debt securities	627	733
Federal government and agency | Recurring
Financial assets at fair value:
Debt securities	627	733
Federal government and agency | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt securities	193	197
Federal government and agency | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt securities	434	536
Federal government and agency | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt securities	0	0
State and local government
Financial assets at fair value:
Debt securities	777	810
State and local government | Recurring
Financial assets at fair value:
Debt securities	777	810
State and local government | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt securities	0	0
State and local government | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt securities	777	810
State and local government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt securities	0	0
Foreign government
Financial assets at fair value:
Debt securities	2,178	2,256
Foreign government | Recurring
Financial assets at fair value:
Debt securities	2,178	2,256
Foreign government | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt securities	0	0
Foreign government | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt securities	2,153	2,228
Foreign government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt securities	25	28
Corporate
Financial assets at fair value:
Debt securities	18,503	19,420
Corporate | Recurring
Financial assets at fair value:
Debt securities	18,503	19,420
Corporate | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt securities	0	0
Corporate | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt securities	17,944	19,063
Corporate | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt securities	559	357
Mortgage and other asset-backed
Financial assets at fair value:
Debt securities	488	536
Mortgage and other asset-backed | Recurring
Financial assets at fair value:
Debt securities	488	536
Mortgage and other asset-backed | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt securities	0	0
Mortgage and other asset-backed | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt securities	321	398
Mortgage and other asset-backed | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt securities	$ 167	$ 138