Claim and Claim Adjustment Expense Reserves, Asbestos and Environmental Pollution Reserves (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Asbestos and Environmental Pollution ("A&EP") Reserves [Abstract]
Fair value of the collateral trust account	$ 2,700		$ 3,700
Asbestos and Environmental Pollution Reserves [Member]
Asbestos and Environmental Pollution ("A&EP") Reserves [Abstract]
Net A&EP claim and allocated claim adjustment expense reserves	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	2,200
Retroactive reinsurance benefit recognized	14	$ 22
Cumulative amounts ceded under the Loss Portfolio Transfer	(3,200)		(3,200)
Remaining unrecognized retroactive reinsurance benefit	$ 378		$ 392