Schedule IV Reinsurance (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 7,521.5	$ 7,434.3	$ 7,494.7
Ceded to Other Companies	223.9	244.2	258.7
Assumed from Other Companies	216.6	241.3	239.5
Net Amount	7,514.2	7,431.4	7,475.5
Percentage of Amount Assumed to Net	2.90%	3.20%	3.20%
Life Insurance in Force
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	782,935.6	740,779.6	675,382.0
Ceded to Other Companies	28,341.1	28,385.4	28,027.1
Assumed from Other Companies	1,141.6	1,850.5	1,895.5
Net Amount	755,736.1	714,244.7	649,250.4
Percentage of Amount Assumed to Net	0.20%	0.30%	0.30%
Life Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	1,868.8	1,810.9	1,757.5
Ceded to Other Companies	146.6	159.1	166.4
Assumed from Other Companies	10.9	12.5	11.8
Net Amount	1,733.1	1,664.3	1,602.9
Percentage of Amount Assumed to Net	0.60%	0.80%	0.70%
Accident and Health Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	5,652.7	5,623.4	5,737.2
Ceded to Other Companies	77.3	85.1	92.3
Assumed from Other Companies	205.7	228.8	227.7
Net Amount	$ 5,781.1	$ 5,767.1	$ 5,872.6
Percentage of Amount Assumed to Net	3.60%	4.00%	3.90%