Reinsurance (Summary Of Direct, Assumed And Ceded Premiums Written And Earned) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reinsurance Disclosures [Abstract]
Direct - Premium written	$ 56,443.6	$ 56,190.3	$ 54,114.3
Assumed - Premium written	197.0	179.9	86.7
Ceded - Premium written	(73.2)	(84.2)	(91.3)
Net premiums - written	56,567.4	56,286.0	54,109.7
Percentage - assumed to net premiums - written	0.30%	0.30%	0.20%
Direct - Premiums earned	56,373.6	55,875.8	53,982.8
Assumed - Premium earned	196.4	178.5	82.7
Ceded - Premium earned	(73.3)	(84.7)	(91.9)
Premiums	$ 56,496.7	$ 55,969.6	$ 53,973.6
Percentage - assumed to net premiums - earned	0.30%	0.30%	0.20%