Fair Value - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	$ 43,984	$ 41,827
Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	25,257	24,944
States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	8,431	6,804
Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	8,670	8,416
Fair Value, Recurring
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	43,351	41,061
Fixed maturities trading	633	766
Total fixed maturities	43,984	41,827
Equity securities	1,292	1,064
Short-term and other	5,871	4,453
Receivables		5
Payable to brokers	(43)	(88)
Fair Value, Recurring | Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	26,250	25,841
Fair Value, Recurring | States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	8,431	6,804
Fair Value, Recurring | Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	8,670	8,416
Fair Value, Recurring | Level 1
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	238	223
Fixed maturities trading	603	766
Total fixed maturities	841	989
Equity securities	762	603
Short-term and other	5,820	4,383
Receivables
Payable to brokers	(43)	(88)
Fair Value, Recurring | Level 1 | Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	238	223
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	40,587	38,642
Fixed maturities trading	30
Total fixed maturities	40,617	38,642
Equity securities	497	441
Short-term and other	51	70
Receivables		5
Payable to brokers
Fair Value, Recurring | Level 2 | Corporate bonds and other
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	24,529	24,340
Fair Value, Recurring | Level 2 | States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	8,386	6,762
Fair Value, Recurring | Level 2 | Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	7,672	7,540
Fair Value, Recurring | Level 3
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	2,526	2,196
Fixed maturities trading
Total fixed maturities	2,526	2,196
Equity securities	33	20
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	1,483	1,278
Fair Value, Recurring | Level 3 | States, municipalities and political subdivisions
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	45	42
Fair Value, Recurring | Level 3 | Asset-backed
Assets and Liabilities Measured at Fair Value [Abstract]
Estimated Fair Value	$ 998	$ 876