Reinsurance Reinsurance - Net Effect of Commutations of Ceded and Cancellations of Assumed Reinsurance Contracts (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Ceded Credit Risk [Line Items]
Commutation gains (losses)	$ 0	$ 0	$ 38	$ 1
Commutations
Ceded Credit Risk [Line Items]
Increase in net unearned premium reserve			5	15
Increase in net par outstanding			336	1,069
Commutation gains (losses)			$ 38	$ 1