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, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Net A&EP adverse development before consideration of LPT	$ 150	$ 178	$ 60
Provision for uncollectible third-party reinsurance on A&EP	(25)	(16)	0
Total additional amounts ceded under LPT	125	162	60
Retroactive reinsurance benefit recognized	(107)	(114)	(68)
Pretax impact of deferred retroactive reinsurance	$ 18	$ 48	$ (8)