Reinsurance (Summary Of Direct, Assumed And Ceded Premiums Written And Earned) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reinsurance Disclosures [Abstract]
Direct - Premium written	$ 72,925.5	$ 68,628.6	$ 65,939.1
Assumed - Premium written	221.8	192.3	174.3
Ceded - Premium written	(95.8)	(108.5)	(92.6)
Net premiums - written	$ 73,051.5	$ 68,712.4	$ 66,020.8
Percentage - assumed to net premiums - written	0.30%	0.30%	0.30%
Direct - Premiums earned	$ 73,259.2	$ 68,304.3	$ 66,038.9
Assumed - Premium earned	221.9	194.0	174.0
Ceded - Premium earned	(96.0)	(108.5)	(93.8)
Premiums	$ 73,385.1	$ 68,389.8	$ 66,119.1
Percentage - assumed to net premiums - earned	0.30%	0.30%	0.30%