Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid and paid losses and LAE	$ 4,320	$ 4,350
Reinsurer concentration risk | Stockholders' equity
Ceded Credit Risk [Line Items]
Concentration risk percentage	1.90%	1.70%
AM Best A minus Or Better Rating | Credit concentration risk | Reinsurance recoverable
Ceded Credit Risk [Line Items]
Concentration risk percentage	60.80%	61.20%
Not Rated | Credit concentration risk | Reinsurance recoverable
Ceded Credit Risk [Line Items]
Concentration risk percentage	38.40%	38.80%
Reinsurance trusts or letters of credit | Not Rated | Credit concentration risk | Reinsurance recoverable
Ceded Credit Risk [Line Items]
Concentration risk percentage	90.00%