Reinsurance - Premium Income Data (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premium Income									$ 7,736.0	$ 7,521.5	$ 7,434.3
Reinsurance Assumed									210.9	216.6	241.3
Reinsurance Ceded									(230.8)	(223.9)	(244.2)
Premiums Earned, Net	1,937.2	1,929.4	1,927.6	1,921.9	1,888.5	1,881.2	1,875.0	1,869.5	7,716.1	7,514.2	7,431.4
Ceded Benefits and Change in Reserves for Future Benefits									$ 591.7	$ 609.2	$ 602.2