Reinsurance (Reinsurance Recoverable on Ceded Reinsurance) (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Disclosures [Abstract]
Reinsurance recoverable on unpaid losses	[1]	$ 14,689	$ 14,014	$ 12,708	$ 10,741
Reinsurance Recoverables on Unpaid Losses, Allowance		251	247
Reinsurance recoverable on paid losses and loss expenses		1,304	1,020
Reinsurance Recoverables on Paid Losses, Allowance		72	74
Net reinsurance recoverable on losses and loss expenses	[2]	15,993	15,034
Reinsurance Recoverables, Allowance		323	321
Reinsurance Recoverable Future Policy Benefits		202	184
Reinsurance Recoverables on Future Policy Benefits, Allowance		$ 4	$ 4

[1] Net of provision for uncollectible reinsurance.

[2] Net of provision for uncollectible reinsurance.