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, 2018	Mar. 31, 2017
Additional amounts ceded under LPT:
Net A&EP adverse development before consideration of LPT	$ 113	$ 60
Provision for uncollectible third-party reinsurance on A&EP	(16)	0
Total additional amounts ceded under LPT	97	60
Retroactive reinsurance benefit recognized	(57)	(40)
Pretax impact of deferred retroactive reinsurance	$ 40	$ 20