Reinsurance (Summary Of Net Premiums Written And Earned By Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	$ 105,726	$ 94,730	$ 85,048
Premiums Earned Net	104,109	94,173	85,421
Commercial and Specialty Business Segment [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	31,745	32,113	30,661
Premiums Earned Net	31,471	31,944	30,532
Government Business [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	72,308	62,617	54,387
Premiums Earned Net	71,188	$ 62,229	$ 54,889
Other Segment [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	1,673
Premiums Earned Net	$ 1,450