Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Property Casualty Insurance Assumed And Ceded Business
Earned premiums	$ 1,669	$ 1,522	$ 4,806	$ 4,460
Incurred loss and loss expenses	1,072	1,143	2,990	3,232
Consolidated Property And Casualty Insurance
Property Casualty Insurance Assumed And Ceded Business
Direct written premiums	1,527	1,386	4,721	4,383
Assumed written premiums	66	55	432	267
Ceded written premiums	(55)	(48)	(208)	(180)
Net written premiums	1,538	1,393	4,945	4,470
Direct earned premiums	1,553	1,439	4,447	4,198
Assumed earned premiums	117	78	327	207
Ceded earned premiums	(74)	(67)	(189)	(163)
Earned premiums	1,596	1,450	4,585	4,242
Direct incurred loss and loss expenses	876	1,039	2,525	2,915
Assumed incurred loss and loss expenses	165	54	297	133
Ceded incurred loss and loss expenses	(53)	(22)	(81)	(40)
Incurred loss and loss expenses	$ 988	$ 1,071	$ 2,741	$ 3,008