Fair Value - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	$ 41,827	$ 40,626
Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	24,944	24,268
States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	6,804	7,392
Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	8,416	7,901
Fair Value, Recurring
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	41,061	40,425
Fixed maturities trading	766	201
Total fixed maturities	41,827	40,626
Equity securities	1,064	1,050
Short-term and other	4,453	4,247
Receivables	5	13
Payable to brokers	(88)	(62)
Fair Value, Recurring | Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	25,841	25,132
Fair Value, Recurring | States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	6,804	7,392
Fair Value, Recurring | Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	8,416	7,901
Fair Value, Recurring | Level 1
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	223	161
Fixed maturities trading	766	201
Total fixed maturities	989	362
Equity securities	603	586
Short-term and other	4,383	4,215
Receivables
Payable to brokers	(88)	(62)
Fair Value, Recurring | Level 1 | Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	223	161
Fair Value, Recurring | Level 1 | States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value
Fair Value, Recurring | Level 1 | Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value
Fair Value, Recurring | Level 2
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	38,642	38,274
Fixed maturities trading
Total fixed maturities	38,642	38,274
Equity securities	441	440
Short-term and other	70	32
Receivables	5	13
Payable to brokers
Fair Value, Recurring | Level 2 | Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	24,340	23,926
Fair Value, Recurring | Level 2 | States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	6,762	7,348
Fair Value, Recurring | Level 2 | Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	7,540	7,000
Fair Value, Recurring | Level 3
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	2,196	1,990
Fixed maturities trading
Total fixed maturities	2,196	1,990
Equity securities	20	24
Short-term and other
Receivables
Payable to brokers
Fair Value, Recurring | Level 3 | Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	1,278	1,045
Fair Value, Recurring | Level 3 | States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	42	44
Fair Value, Recurring | Level 3 | Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	$ 876	$ 901