Schedule IV Reinsurance (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reinsurance Premiums for Insurance Companies, by Product Segment
Direct Premiums, Life Insurance in Force	$ 990,371.0				$ 937,300.8				$ 990,371.0	$ 937,300.8	$ 887,857.2
Ceded Premiums, Life Insurance in Force	41,669.8				40,902.8				41,669.8	40,902.8	36,545.0
Assumed Premiums, Life Insurance in Force	1,018.4				1,089.3				1,018.4	1,089.3	1,045.8
Premiums, Net, Life Insurance in Force	$ 949,719.6				$ 897,487.3				$ 949,719.6	$ 897,487.3	$ 852,358.0
Life Insurance in Force Premiums, Percentage Assumed to Net	0.10%				0.10%				0.10%	0.10%	0.10%
Gross Amount									$ 9,576.3	$ 9,171.1	$ 8,677.2
Reinsurance Ceded									327.2	327.6	310.1
Assumed Premiums Earned									116.5	142.6	230.0
Net Amount	$ 2,352.6	$ 2,331.2	$ 2,343.1	$ 2,338.7	$ 2,263.3	$ 2,251.8	$ 2,221.0	$ 2,250.0	$ 9,365.6	$ 8,986.1	$ 8,597.1
Percentage of Amount Assumed to Net									1.20%	1.60%	2.70%
Life Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 2,549.7	$ 2,442.1	$ 2,272.7
Reinsurance Ceded									137.2	138.2	135.7
Assumed Premiums Earned									8.2	8.7	10.2
Net Amount									$ 2,420.7	$ 2,312.6	$ 2,147.2
Percentage of Amount Assumed to Net									0.30%	0.40%	0.50%
Accident, Health, and Other Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 7,026.6	$ 6,729.0	$ 6,404.5
Reinsurance Ceded									190.0	189.4	174.4
Assumed Premiums Earned									108.3	133.9	219.8
Net Amount									$ 6,944.9	$ 6,673.5	$ 6,449.9
Percentage of Amount Assumed to Net									1.60%	2.00%	3.40%