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	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Net A&EP adverse development before consideration of LPT	$ 0	$ 113
Provision for uncollectible third-party reinsurance on A&EP	0	(16)
Total additional amounts ceded under LPT	0	97
Retroactive reinsurance benefit recognized	(22)	(57)
Pretax impact of deferred retroactive reinsurance	$ (22)	$ 40