Claim and Claim Adjustment Expense Reserves, Asbestos and Environmental Pollution Reserves (Details) - A&EP Reserves [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2010	Dec. 31, 2020
Asbestos and Environmental Pollution Reserves ("A&EP") [Abstract]
Net A&EP claim and allocated claim adjustment expense reserves					$ 1,600
Aggregate limit under A&EP Loss Portfolio Transfer					4,000
Ceded A&EP claim and allocated claim adjustment expense reserves 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 book value of billed third party reinsurance receivables transferred to NICO under A&EP loss portfolio transfer					215
Total consideration					$ 2,200
Retroactive reinsurance benefit recognized	$ 8	$ 9	$ 30	$ 43
Cumulative amounts ceded under loss portfolio transfer	3,300		3,300			$ 3,300
Unrecognized deferred retroactive reinsurance benefit	368		368			$ 398
Fair value of collateral trust account	$ 2,900		$ 2,900