Reinsurance - Premium Income Data (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premium Income									$ 7,521.5	$ 7,434.3	$ 7,494.7
Reinsurance Assumed									216.6	241.3	239.5
Reinsurance Ceded									(223.9)	(244.2)	(258.7)
Net Premium Income	1,888.5	1,881.2	1,875.0	1,869.5	1,868.2	1,850.2	1,849.8	1,863.2	7,514.2	7,431.4	7,475.5
Ceded Benefits and Change in Reserves for Future Benefits									$ 609.2	$ 602.2	$ 604.2