Reinsurance Reinsurance - Net Effect of Commutations of Ceded and Cancellations of Assumed Reinsurance Contracts (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Ceded Credit Risk [Line Items]
Commutation gains (losses)	$ 38	$ 1	$ 38	$ 1
Commutations [Member]
Ceded Credit Risk [Line Items]
Increase in net unearned premium reserve	5	15	5	15
Increase in net par outstanding	336	1,069	336	1,069
Commutation gains (losses)	$ 38	$ 1	$ 38	$ 1