Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Property Casualty Insurance Assumed And Ceded Business
Earned premiums	$ 1,482	$ 1,384	$ 2,938	$ 2,717
Incurred loss and loss expenses	1,086	936	2,089	1,796
Consolidated Property and Casualty Insurance Entity
Property Casualty Insurance Assumed And Ceded Business
Direct written premiums	1,539	1,472	2,997	2,807
Assumed written premiums	101	75	212	162
Ceded written premiums	(81)	(71)	(132)	(112)
Net written premiums	1,559	1,476	3,077	2,857
Direct earned premiums	1,388	1,319	2,759	2,585
Assumed earned premiums	63	50	129	93
Ceded earned premiums	(48)	(52)	(96)	(94)
Earned premiums	1,403	1,317	2,792	2,584
Direct incurred loss and loss expenses	976	866	1,876	1,653
Assumed incurred loss and loss expenses	45	25	79	50
Ceded incurred loss and loss expenses	(14)	(28)	(18)	(50)
Incurred loss and loss expenses	$ 1,007	$ 863	$ 1,937	$ 1,653