Schedule IV Reinsurance (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment
Direct Premiums, Life Insurance in Force	$ 887,857.2				$ 854,783.9				$ 887,857.2	$ 854,783.9	$ 827,515.2
Ceded Premiums, Life Insurance in Force	36,545.0				31,882.5				36,545.0	31,882.5	24,062.6
Assumed Premiums, Life Insurance in Force	1,045.8				1,004.7				1,045.8	1,004.7	993.5
Premiums, Net, Life Insurance in Force	$ 852,358.0				$ 823,906.1				$ 852,358.0	$ 823,906.1	$ 804,446.1
Life Insurance in Force Premiums, Percentage Assumed to Net	0.10%				0.10%				0.10%	0.10%	0.10%
Gross Amount									$ 8,677.2	$ 8,385.0	$ 8,151.9
Reinsurance Ceded									310.1	263.6	272.3
Assumed Premiums Earned									230.0	236.3	202.8
Net Amount	$ 2,158.4	$ 2,153.6	$ 2,142.2	$ 2,142.9	$ 2,099.2	$ 2,089.4	$ 2,081.6	$ 2,087.5	$ 8,597.1	$ 8,357.7	$ 8,082.4
Percentage of Amount Assumed to Net									2.70%	2.80%	2.50%
Life Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 2,272.7	$ 2,202.6	$ 2,150.1
Reinsurance Ceded									135.7	151.3	176.3
Assumed Premiums Earned									10.2	9.7	9.4
Net Amount									$ 2,147.2	$ 2,061.0	$ 1,983.2
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									$ 6,404.5	$ 6,182.4	$ 6,001.8
Reinsurance Ceded									174.4	112.3	96.0
Assumed Premiums Earned									219.8	226.6	193.4
Net Amount									$ 6,449.9	$ 6,296.7	$ 6,099.2
Percentage of Amount Assumed to Net									3.40%	3.60%	3.20%