Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Property Casualty Insurance Assumed And Ceded Business
Earned premiums	$ 1,593	$ 1,482	$ 3,137	$ 2,938
Incurred loss and loss expenses	915	1,086	1,918	2,089
Consolidated Property And Casualty Insurance
Property Casualty Insurance Assumed And Ceded Business
Direct written premiums	1,649	1,539	3,194	2,997
Assumed written premiums	161	101	366	212
Ceded written premiums	(96)	(81)	(153)	(132)
Net written premiums	1,714	1,559	3,407	3,077
Direct earned premiums	1,465	1,388	2,894	2,759
Assumed earned premiums	109	63	210	129
Ceded earned premiums	(60)	(48)	(115)	(96)
Earned premiums	1,514	1,403	2,989	2,792
Direct incurred loss and loss expenses	781	976	1,649	1,876
Assumed incurred loss and loss expenses	54	45	132	79
Ceded incurred loss and loss expenses	(5)	(14)	(28)	(18)
Incurred loss and loss expenses	$ 830	$ 1,007	$ 1,753	$ 1,937