Schedule IV Reinsurance (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reinsurance Premiums for Insurance Companies, by Product Segment
Direct Premiums, Life Insurance in Force	$ 827,515.2				$ 790,952.1				$ 827,515.2	$ 790,952.1	$ 781,495.9
Ceded Premiums, Life Insurance in Force	24,062.6				24,416.7				24,062.6	24,416.7	25,904.7
Assumed Premiums, Life Insurance in Force	993.5				999.7				993.5	999.7	1,026.2
Premiums, Net, Life Insurance in Force	$ 804,446.1				$ 767,535.1				$ 804,446.1	$ 767,535.1	$ 756,617.4
Life Insurance in Force Premiums, Percentage Assumed to Net	0.10%				0.10%				0.10%	0.10%	0.10%
Gross Amount									$ 8,151.9	$ 7,899.3	$ 7,777.3
Reinsurance Ceded									272.3	291.9	355.8
Assumed Premiums Earned									202.8	189.8	203.2
Net Amount	$ 2,037.8	$ 2,020.8	$ 2,017.5	$ 2,006.3	$ 1,967.9	$ 1,947.2	$ 1,943.6	$ 1,938.5	$ 8,082.4	$ 7,797.2	$ 7,624.7
Percentage of Amount Assumed to Net									2.50%	2.40%	2.70%
Life Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 2,150.1	$ 2,056.4	$ 2,018.7
Reinsurance Ceded									176.3	194.8	253.6
Assumed Premiums Earned									9.4	9.6	10.0
Net Amount									$ 1,983.2	$ 1,871.2	$ 1,775.1
Percentage of Amount Assumed to Net									0.50%	0.50%	0.60%
Accident, Health, and Other Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 6,001.8	$ 5,842.9	$ 5,758.6
Reinsurance Ceded									96.0	97.1	102.2
Assumed Premiums Earned									193.4	180.2	193.2
Net Amount									$ 6,099.2	$ 5,926.0	$ 5,849.6
Percentage of Amount Assumed to Net									3.20%	3.00%	3.30%