Expected Credit Losses - Uncollectible Reinsurance and Insurance Receivables (Summary of Outstanding Amount of Voluntary Reinsuranece Receivables) (Details) $ in Millions	Mar. 31, 2020 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
Reinsurance receivables	$ 3,525
A- to A
Financing Receivable, Credit Quality Indicator [Line Items]
Reinsurance receivables	2,662
B- to B
Financing Receivable, Credit Quality Indicator [Line Items]
Reinsurance receivables	859
Insolvent
Financing Receivable, Credit Quality Indicator [Line Items]
Reinsurance receivables	$ 4