Reinsurance (Summary Of Net Premiums Written And Earned By Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	$ 78,338.0	$ 73,051.5	$ 68,712.4
Premiums Earned Net	78,860.1	73,385.1	68,389.8
Commercial and Specialty Business Segment [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	33,355.6	33,016.9	35,084.7
Premiums Earned Net	33,831.5	33,078.0	35,045.2
Government Business [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	44,982.4	40,034.6	33,627.7
Premiums Earned Net	$ 45,028.6	$ 40,307.1	$ 33,344.6