Schedule IV Reinsurance (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance Premiums for Insurance Companies, by Product Segment
Direct Premiums, Life Insurance in Force	$ 854,783.9				$ 827,515.2				$ 854,783.9	$ 827,515.2	$ 790,952.1
Ceded Premiums, Life Insurance in Force	31,882.5				24,062.6				31,882.5	24,062.6	24,416.7
Assumed Premiums, Life Insurance in Force	1,004.7				993.5				1,004.7	993.5	999.7
Premiums, Net, Life Insurance in Force	$ 823,906.1				$ 804,446.1				$ 823,906.1	$ 804,446.1	$ 767,535.1
Life Insurance in Force Premiums, Percentage Assumed to Net	0.10%				0.10%				0.10%	0.10%	0.10%
Gross Amount									$ 8,385.0	$ 8,151.9	$ 7,899.3
Reinsurance Ceded									263.6	272.3	291.9
Assumed Premiums Earned									236.3	202.8	189.8
Net Amount	$ 2,099.2	$ 2,089.4	$ 2,081.6	$ 2,087.5	$ 2,037.8	$ 2,020.8	$ 2,017.5	$ 2,006.3	$ 8,357.7	$ 8,082.4	$ 7,797.2
Percentage of Amount Assumed to Net									2.80%	2.50%	2.40%
Life Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 2,202.6	$ 2,150.1	$ 2,056.4
Reinsurance Ceded									151.3	176.3	194.8
Assumed Premiums Earned									9.7	9.4	9.6
Net Amount									$ 2,061.0	$ 1,983.2	$ 1,871.2
Percentage of Amount Assumed to Net									0.50%	0.50%	0.50%
Accident, Health, and Other Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 6,182.4	$ 6,001.8	$ 5,842.9
Reinsurance Ceded									112.3	96.0	97.1
Assumed Premiums Earned									226.6	193.4	180.2
Net Amount									$ 6,296.7	$ 6,099.2	$ 5,926.0
Percentage of Amount Assumed to Net									3.60%	3.20%	3.00%