Reinsurance (Summary Of Direct, Assumed And Ceded Premiums Written And Earned) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Direct - Premium written	$ 78,200.4	$ 72,925.5	$ 68,628.6
Assumed - Premium written	217.4	221.8	192.3
Ceded - Premium written	(79.8)	(95.8)	(108.5)
Net premiums - written	$ 78,338.0	$ 73,051.5	$ 68,712.4
Percentage - assumed to net premiums - written	0.30%	0.30%	0.30%
Direct - Premiums earned	$ 78,726.2	$ 73,259.2	$ 68,304.3
Assumed - Premium earned	217.3	221.9	194.0
Ceded - Premium earned	(83.4)	(96.0)	(108.5)
Premiums	$ 78,860.1	$ 73,385.1	$ 68,389.8
Percentage - assumed to net premiums - earned	0.30%	0.30%	0.30%