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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Additional amounts ceded under LPT:
Net A&EP adverse development before consideration of LPT	$ 0	$ 0	$ 113	$ 60
Provision for uncollectible third-party reinsurance on A&EP	0	0	(16)	0
Total additional amounts ceded under LPT	0	0	97	60
Retroactive reinsurance benefit recognized	(15)	(3)	(72)	(43)
Pretax impact of deferred retroactive reinsurance	$ (15)	$ (3)	$ 25	$ 17