Reinsurance (Reinsurance Recoverable on Ceded Reinsurance) (Details) - USD ($) $ in Millions		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Reinsurance recoverable on unpaid losses	[1]	$ 14,014	[2]	$ 12,708	[2]	$ 10,741	$ 11,307
Reinsurance recoverable on paid losses and loss expenses	[2]	1,020		869
Net reinsurance recoverable on losses and loss expenses	[2]	15,034		13,577
Reinsurance Recoverable Future Policy Benefits	[2]	$ 184		$ 182

[1] (1) Net of provision for uncollectible reinsurance.

[2] (1) Net of a provision for uncollectible reinsurance.