Reinsurance Reinsurance (Written Consolidated Property Casualty Insurance Premiums On Assumed and Ceded Business) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-17, Insurance Companies, Reinsurance
Direct premiums earned	$ 5,985	$ 5,681	$ 5,251
Assumed premiums earned	285	199	149
Ceded premiums earned	(290)	(276)	(230)
Incurred loss and loss expenses	4,134	3,638	3,490
Consolidated Property And Casualty Insurance Entity
SEC Schedule, 12-17, Insurance Companies, Reinsurance
Direct premiums written	5,756	5,477	5,018
Assumed premiums written	335	244	173
Ceded premiums written	(227)	(205)	(161)
Net written premiums	5,864	5,516	5,030
Direct premiums earned	5,623	5,340	4,931
Assumed premiums earned	285	199	149
Ceded premiums earned	(217)	(205)	(160)
Earned premiums	5,691	5,334	4,920
Direct incurred loss and loss expenses	3,699	3,402	3,188
Assumed incurred loss and loss expenses	184	117	125
Ceded incurred loss and loss expenses	(46)	(167)	(90)
Incurred loss and loss expenses	$ 3,837	$ 3,352	$ 3,223