Reinsurance (Reinsurance Recoverable on Ceded Reinsurance) (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reinsurance Disclosures [Abstract]
Reinsurance recoverable on unpaid losses	[1]	$ 14,647	$ 14,181	$ 14,689	$ 14,014
Reinsurance Recoverables on Unpaid Losses, Allowance		257	240
Reinsurance recoverable on paid losses and loss expenses		945	1,000
Reinsurance Recoverables on Paid Losses, Allowance		57	76
Net reinsurance recoverable on losses and loss expenses	[2]	15,592	15,181
Reinsurance Recoverable, Allowance for Credit Loss		314	316
Reinsurance Recoverable Future Policy Benefits		206	197
Reinsurance Recoverables on Future Policy Benefits, Allowance		$ 5	$ 4

[1] Net of valuation allowance for uncollectible reinsurance.

[2] Net of valuation allowance for uncollectible reinsurance.