Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($)		3 Months Ended			6 Months Ended		13 Months Ended
	Feb. 05, 2021	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Feb. 05, 2021	Dec. 31, 2020	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Unusual or infrequent item, or both, net of insurance proceeds		$ 54,000,000		$ 301,000,000	$ 179,000,000	$ 376,000,000
Retroactive reinsurance benefit recognized		12,000,000		20,000,000	22,000,000	34,000,000
Funds held under reinsurance agreements, liability	$ 690,000,000	690,000,000			690,000,000		$ 690,000,000
Retroactive reinsurance agreement, maximum	1,000,000,000						1,000,000,000
Reinsurance premiums paid	$ 697,000,000						$ 64,000,000
Reinsurance, loss on uncollectible accounts in period, amount			$ 12,000,000
Retroactive reinsurance agreement, amount available		$ 310,000,000			$ 310,000,000
Collateral trust account, percentage of outstanding reserves		105.00%			105.00%
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development		$ 40,000,000		50,000,000	$ 40,000,000	50,000,000
COVID-19
Liability for Claims and Claims Adjustment Expense [Line Items]
Unusual or infrequent item, or both, net of insurance proceeds				182,000,000		195,000,000
Civil Unrest
Liability for Claims and Claims Adjustment Expense [Line Items]
Unusual or infrequent item, or both, net of insurance proceeds				61,000,000		61,000,000
Weather Related Events
Liability for Claims and Claims Adjustment Expense [Line Items]
Unusual or infrequent item, or both, net of insurance proceeds				$ 58,000,000		$ 120,000,000
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									$ 1,600,000,000
Aggregate limit under A&EP Loss Portfolio Transfer									4,000,000,000
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									1,200,000,000
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer									2,000,000,000
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer									215,000,000
Total consideration paid to NICO under AEP Loss Portfolio Transfer									$ 2,200,000,000
Cumulative amounts ceded under AEP Loss Portfolio Transfer		3,300,000,000			3,300,000,000			$ 3,300,000,000
Deferred reinsurance benefit yet to be recognized		376,000,000			376,000,000			$ 398,000,000
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer		$ 2,900,000,000			$ 2,900,000,000