Allowance for Expected Credit Losses (Details) - Narrative - Reinsurer Concentration Risk [Member] $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)
Reinsurance Recoverables [Member] | Reinsurer Group Rated by A.M. Best [Member] | A.M. Best, Rating Provided [Member]
Reinsurance recoverables, credit quality indicator
Reinsurance recoverables, after deducting mandatory pools and associations and before allowances for expected credit losses	$ 5,820
Concentration risk percentage	88.00%
Reinsurance Recoverables [Member] | Reinsurer Group Not Rated by A.M. Best [Member]
Reinsurance recoverables, credit quality indicator
Concentration risk percentage	12.00%
Reinsurance Recoverables [Member] | Captive Insurance Companies [Member]
Reinsurance recoverables, credit quality indicator
Concentration risk percentage	6.00%
Reinsurance Recoverables [Member] | Voluntary Pools [Member]
Reinsurance recoverables, credit quality indicator
Concentration risk percentage	1.00%
Reinsurance Recoverables [Member] | Other Companies [Member]
Reinsurance recoverables, credit quality indicator
Concentration risk percentage	5.00%
Reinsurance Recoverables Rated by A.M. Best [Member] | Reinsurer Group Rated by A.M. Best [Member] | A.M. Best, A- Rating Or Better [Member]
Reinsurance recoverables, credit quality indicator
Concentration risk percentage	93.00%