Reinsurance (Reinsurance Recoverable on Ceded Reinsurance) (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Disclosures [Abstract]
Reinsurance recoverable on unpaid losses	[1]	$ 14,181	$ 14,689	$ 14,014	$ 12,708
Reinsurance Recoverables on Unpaid Losses, Allowance		240	251
Reinsurance recoverable on paid losses and loss expenses		1,000	1,304
Reinsurance Recoverables on Paid Losses, Allowance		76	72
Net reinsurance recoverable on losses and loss expenses	[2]	15,181	15,993
Reinsurance Recoverable, Allowance for Credit Loss		316	323
Reinsurance Recoverable Future Policy Benefits		197	202
Reinsurance Recoverables on Future Policy Benefits, Allowance		$ 4	$ 4

[1] Net of provision for uncollectible reinsurance.

[2] Net of provision for uncollectible reinsurance.