Reinsurance (Summary Of Net Premiums Written And Earned By Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	$ 94,730	$ 85,048	$ 84,205
Premiums Earned Net	94,173	85,421	83,648
Commercial and Specialty Business Segment [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	32,113	30,661	35,382
Premiums Earned Net	31,944	30,532	35,503
Government Business [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	62,617	54,387	48,823
Premiums Earned Net	$ 62,229	$ 54,889	$ 48,145