Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2025	Dec. 31, 2014	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Largest recoverable from single reinsurer	$ 6,497		$ 6,497		$ 6,381
Pretax (favorable) unfavorable premium development, excluding Life & Group	91	$ 108	191	$ 191
Reinsurance premiums paid	0	0	9	0
Retroactive reinsurance benefit recognized	24	46	8	25
Unusual or Infrequent Item, or Both, Net of Insurance Proceeds	60	62	148	159
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	97	$ 112	97	$ 134
Asbestos and Environmental Reserves
Liability for Claims and Claims Adjustment Expense [Line Items]
Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO							$ 1,600
Aggregate limit under A&EP Loss Portfolio Transfer							4,000
A&EP claim and allocated claim adjustment expense reserves ceded under existing third party reinsurance contracts transferred to NICO under A&EP Loss Portfolio Transfer							1,200
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer							2,000
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer							215
Total consideration paid to NICO under AEP Loss Portfolio Transfer						$ 2,200	$ 2,200
Cumulative amounts ceded under AEP Loss Portfolio Transfer	3,900		3,900
Deferred reinsurance benefit yet to be recognized	424		424		$ 470
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer	$ 1,800		$ 1,800