Fair Value - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	$ 40,605	$ 40,425
Fixed maturities trading	235	201
Total fixed maturities	40,840	40,626
Equity securities	1,104	1,050
Short-term and other	5,222	4,247
Receivables	12	13
Payable to brokers	(38)	(62)
Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	25,237	25,132
States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	7,288	7,392
Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	8,080	7,901
Level 1
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	189	161
Fixed maturities trading	235	201
Total fixed maturities	424	362
Equity securities	654	586
Short-term and other	5,203	4,215
Receivables
Payable to brokers	(38)	(62)
Level 1 | Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	189	161
Level 1 | States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities
Level 1 | Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities
Level 2
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	38,420	38,274
Fixed maturities trading
Total fixed maturities	38,420	38,274
Equity securities	439	440
Short-term and other	19	32
Receivables	12	13
Payable to brokers
Level 2 | Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	23,966	23,926
Level 2 | States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	7,245	7,348
Level 2 | Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	7,209	7,000
Level 3
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	1,996	1,990
Fixed maturities trading
Total fixed maturities	1,996	1,990
Equity securities	11	24
Short-term and other
Receivables
Payable to brokers
Level 3 | Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	1,082	1,045
Level 3 | States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	43	44
Level 3 | Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Debt securities	$ 871	$ 901