Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($)		3 Months Ended		13 Months Ended
	Feb. 05, 2021	Mar. 31, 2021	Mar. 31, 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		$ 125,000,000	$ 75,000,000
Retroactive reinsurance benefit recognized		10,000,000	14,000,000
Funds held under reinsurance agreements, liability	$ 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
Percentage of outstanding reserves	105.00%			105.00%
Retroactive reinsurance agreement, amount remaining		310,000,000
Weather Related Events
Liability for Claims and Claims Adjustment Expense [Line Items]
Unusual or infrequent item, or both, net of insurance proceeds			62,000,000
COVID-19
Liability for Claims and Claims Adjustment Expense [Line Items]
Unusual or infrequent item, or both, net of insurance proceeds			$ 13,000,000
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
Deferred reinsurance benefit yet to be recognized		388,000,000			398,000,000
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer		$ 4,600,000,000			$ 4,200,000,000