Reinsurance Reinsurance (Written Consolidated Property Casualty Insurance Premiums On Assumed and Ceded Business) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-17, Insurance Companies, Reinsurance
Direct premiums earned	$ 5,681	$ 5,251	$ 5,052
Assumed premiums earned	199	149	132
Ceded premiums earned	276	230	230
Incurred loss and loss expenses	3,638	3,490	3,390
Consolidated Property and Casualty Insurance Entity
SEC Schedule, 12-17, Insurance Companies, Reinsurance
Direct premiums written	5,477	5,018	4,854
Assumed premiums written	244	173	125
Ceded premiums written	(205)	(161)	(139)
Net written premiums	5,516	5,030	4,840
Direct premiums earned	5,340	4,931	4,752
Assumed premiums earned	199	149	132
Ceded premiums earned	(205)	(160)	(162)
Earned premiums	5,334	4,920	4,722
Direct incurred loss and loss expenses	3,402	3,188	2,961
Assumed incurred loss and loss expenses	117	125	113
Ceded incurred loss and loss expenses	(167)	(90)	64
Incurred loss and loss expenses	$ 3,352	$ 3,223	$ 3,138