Reinsurance (Ceded Reinsurance Receivable Assets) (Details) - USD ($) $ in Millions	6 Months Ended	9 Months Ended
	Jun. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 1,093		$ 983
Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 1,093		983
Reinsurer Concentration Risk	100.00%	100.00%
Other Reinsurers [Member] | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 209		182
Reinsurer Concentration Risk	19.20%	18.50%
AM Best, A+ Rating [Member] | Reinsurer A [Member] | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 476		420
Reinsurer Concentration Risk	43.50%	42.70%
AM Best, A+ Rating [Member] | Reinsurer B [Member] | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 225		216
Reinsurer Concentration Risk	20.60%	22.00%
AM Best, A+ Rating [Member] | Reinsurer E [Member] | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 50		46
Reinsurer Concentration Risk	4.60%	4.70%
AM Best, A Rating [Member] | Reinsurer C [Member] | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 67		64
Reinsurer Concentration Risk	6.10%	6.50%
AM Best, A++ Rating [Member] | Reinsurer D [Member] | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 66		$ 55
Reinsurer Concentration Risk	6.00%	5.60%