Reinsurance (Summary Of Net Premiums Written And Earned By Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	$ 73,051.5	$ 68,712.4	$ 66,020.8
Premiums Earned Net	73,385.1	68,389.8	66,119.1
Commercial and Specialty Business [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	33,016.9	35,084.7	35,733.9
Premiums Earned Net	33,078.0	35,045.2	35,772.0
Government Business [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	40,034.6	33,627.7	30,286.9
Premiums Earned Net	40,307.1	33,344.6	30,347.1
Other [Member]
Net Premiums Written And Earned By Segment [Line Items]
Premiums Written Net	0.0	0.0	0.0
Premiums Earned Net	$ 0.0	$ 0.0	$ 0.0