Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Property Casualty Insurance Assumed And Ceded Business
Earned premiums	$ 1,544	$ 1,456
Incurred loss and loss expenses	1,003	1,003
Consolidated Property And Casualty Insurance
Property Casualty Insurance Assumed And Ceded Business
Direct written premiums	1,545	1,458
Assumed written premiums	205	111
Ceded written premiums	(57)	(51)
Net written premiums	1,693	1,518
Direct earned premiums	1,429	1,371
Assumed earned premiums	101	66
Ceded earned premiums	(55)	(48)
Earned premiums	1,475	1,389
Direct incurred loss and loss expenses	868	900
Assumed incurred loss and loss expenses	78	34
Ceded incurred loss and loss expenses	(23)	(4)
Incurred loss and loss expenses	$ 923	$ 930