Claim and Claim Adjustment Expense Reserves (Impact of Loss Portfolio Transfer on the Consolidated Statement of Operations) (Details) - Asbestos and Environmental Reserves - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Net A&EP adverse development before consideration of LPT	$ 125	$ 150	$ 178
Provision for uncollectible third-party reinsurance on A&EP	(25)	(25)	(16)
Total additional amounts ceded under LPT	100	125	162
Retroactive reinsurance benefit recognized	(94)	(107)	(114)
Pretax impact of deferred retroactive reinsurance	$ 6	$ 18	$ 48