Reinsurance (Summary Of Net Premiums Written And Earned By Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	$ 84,205.2	$ 78,338.0	$ 73,051.5
Premiums Earned Net	83,647.7	78,860.1	73,385.1
Commercial and Specialty Business Segment [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	35,683.1	33,355.6	33,016.9
Premiums Earned Net	35,804.3	33,831.5	33,078.0
Government Business [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	48,522.1	44,982.4	40,034.6
Premiums Earned Net	$ 47,843.4	$ 45,028.6	$ 40,307.1