Fair Value Measurements - Financial Assets and Financial Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financial assets at fair value:
Debt securities	$ 24,673	$ 23,755
Equity securities	124	64
Financial liabilities at fair value
Unfunded commitments	50
Recurring
Financial assets at fair value:
Debt securities	24,673	23,755
Equity securities	167	111
Short-term investments	458	423
Real estate funds priced at NAV as a practical expedient	165	184
Recurring | Other Derivatives
Financial assets at fair value:
Derivative assets	104	83
Financial liabilities at fair value
Derivative liabilities	21	18
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt securities	183	197
Equity securities	1	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	23,575	23,035
Equity securities	135	72
Short-term investments	458	423
Significant Other Observable Inputs (Level 2) | Recurring | Other Derivatives
Financial assets at fair value:
Derivative assets	104	83
Financial liabilities at fair value
Derivative liabilities	21	18
Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt securities	915	523
Equity securities	31	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	634	733
Federal government and agency | Recurring
Financial assets at fair value:
Debt securities	634	733
Federal government and agency | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt securities	183	197
Federal government and agency | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt securities	451	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	713	810
State and local government | Recurring
Financial assets at fair value:
Debt securities	713	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	713	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	2,402	2,256
Foreign government | Recurring
Financial assets at fair value:
Debt securities	2,402	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	2,381	2,228
Foreign government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt securities	21	28
Corporate
Financial assets at fair value:
Debt securities	20,409	19,420
Corporate | Recurring
Financial assets at fair value:
Debt securities	20,409	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	19,662	19,063
Corporate | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt securities	747	357
Mortgage and other asset-backed
Financial assets at fair value:
Debt securities	515	536
Mortgage and other asset-backed | Recurring
Financial assets at fair value:
Debt securities	515	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	368	398
Mortgage and other asset-backed | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt securities	$ 147	$ 138