Expected Credit Losses - Uncollectible Reinsurance and Insurance Receivables (Summary of Outstanding Amount of Voluntary Reinsurance Receivables) (Details) $ in Millions	Sep. 30, 2020 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
Reinsurance receivables	$ 3,607
A- to A
Financing Receivable, Credit Quality Indicator [Line Items]
Reinsurance receivables	2,729
B- to B
Financing Receivable, Credit Quality Indicator [Line Items]
Reinsurance receivables	874
Insolvent
Financing Receivable, Credit Quality Indicator [Line Items]
Reinsurance receivables	$ 4