Expected Credit Losses - Reinsurance and Insurance Receivables (Details) $ in Millions	Mar. 31, 2020 USD ($)
Voluntary Reinsurance Receivable [Abstract]
Allowance for doubtful accounts for reinsurance receivables	$ 23
Voluntary reinsurance outstanding balance	3,525	[1]
Allowance for doubtful accounts for insurance receivables	30
A- to A++ [Member]
Voluntary Reinsurance Receivable [Abstract]
Voluntary reinsurance outstanding balance	2,662
B- to B++ [Member]
Voluntary Reinsurance Receivable [Abstract]
Voluntary reinsurance outstanding balance	859
Insolvent [Member]
Voluntary Reinsurance Receivable [Abstract]
Voluntary reinsurance outstanding balance	$ 4

[1] Expected credit losses for legacy A&EP receivables are ceded to NICO and the reinsurance limit on the LPT has not been exhausted, therefore no allowance is recorded for these receivables and they are excluded from the table above. See Note 5 for more information on the LPT. Also excluded are receivables from involuntary pools.