Schedule IV Reinsurance (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 7,736.0	$ 7,521.5	$ 7,434.3
Ceded to Other Companies	230.8	223.9	244.2
Assumed from Other Companies	210.9	216.6	241.3
Net Amount	7,716.1	7,514.2	7,431.4
Percentage of Amount Assumed to Net	2.70%	2.90%	3.20%
Life Insurance in Force
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	832,587.5	782,935.6	740,779.6
Ceded to Other Companies	28,658.7	28,341.1	28,385.4
Assumed from Other Companies	1,073.8	1,141.6	1,850.5
Net Amount	805,002.6	755,736.1	714,244.7
Percentage of Amount Assumed to Net	0.10%	0.20%	0.30%
Life Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	1,979.1	1,868.8	1,810.9
Ceded to Other Companies	141.4	146.6	159.1
Assumed from Other Companies	10.3	10.9	12.5
Net Amount	1,848.0	1,733.1	1,664.3
Percentage of Amount Assumed to Net	0.60%	0.60%	0.80%
Accident and Health Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	5,756.9	5,652.7	5,623.4
Ceded to Other Companies	89.4	77.3	85.1
Assumed from Other Companies	200.6	205.7	228.8
Net Amount	$ 5,868.1	$ 5,781.1	$ 5,767.1
Percentage of Amount Assumed to Net	3.40%	3.60%	4.00%