Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Property Casualty Insurance Assumed And Ceded Business
Earned premiums	$ 1,522	$ 1,446	$ 4,460	$ 4,163
Incurred loss and loss expenses	1,143	932	3,232	2,728
Consolidated Property and Casualty Insurance Entity
Property Casualty Insurance Assumed And Ceded Business
Direct written premiums	1,386	1,361	4,383	4,168
Assumed written premiums	55	41	267	203
Ceded written premiums	(48)	(50)	(180)	(162)
Net written premiums	1,393	1,352	4,470	4,209
Direct earned premiums	1,439	1,386	4,198	3,971
Assumed earned premiums	78	52	207	145
Ceded earned premiums	(67)	(62)	(163)	(156)
Earned premiums	1,450	1,376	4,242	3,960
Direct incurred loss and loss expenses	1,039	855	2,915	2,508
Assumed incurred loss and loss expenses	54	37	133	87
Ceded incurred loss and loss expenses	(22)	(28)	(40)	(78)
Incurred loss and loss expenses	$ 1,071	$ 864	$ 3,008	$ 2,517