Schedule IV Reinsurance (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Premiums for Insurance Companies, by Product Segment
Direct Premiums, Life Insurance in Force	$ 937,300.8				$ 887,857.2				$ 937,300.8	$ 887,857.2	$ 854,783.9
Ceded Premiums, Life Insurance in Force	40,902.8				36,545.0				40,902.8	36,545.0	31,882.5
Assumed Premiums, Life Insurance in Force	1,089.3				1,045.8				1,089.3	1,045.8	1,004.7
Premiums, Net, Life Insurance in Force	$ 897,487.3				$ 852,358.0				$ 897,487.3	$ 852,358.0	$ 823,906.1
Life Insurance in Force Premiums, Percentage Assumed to Net	0.10%				0.10%				0.10%	0.10%	0.10%
Gross Amount									$ 9,171.1	$ 8,677.2	$ 8,385.0
Reinsurance Ceded									327.6	310.1	263.6
Assumed Premiums Earned									142.6	230.0	236.3
Net Amount	$ 2,263.3	$ 2,251.8	$ 2,221.0	$ 2,250.0	$ 2,158.4	$ 2,153.6	$ 2,142.2	$ 2,142.9	$ 8,986.1	$ 8,597.1	$ 8,357.7
Percentage of Amount Assumed to Net									1.60%	2.70%	2.80%
Life Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 2,442.1	$ 2,272.7	$ 2,202.6
Reinsurance Ceded									138.2	135.7	151.3
Assumed Premiums Earned									8.7	10.2	9.7
Net Amount									$ 2,312.6	$ 2,147.2	$ 2,061.0
Percentage of Amount Assumed to Net									0.40%	0.50%	0.50%
Accident, Health, and Other Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 6,729.0	$ 6,404.5	$ 6,182.4
Reinsurance Ceded									189.4	174.4	112.3
Assumed Premiums Earned									133.9	219.8	226.6
Net Amount									$ 6,673.5	$ 6,449.9	$ 6,296.7
Percentage of Amount Assumed to Net									2.00%	3.40%	3.60%