Schedule IV Reinsurance (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reinsurance Premiums for Insurance Companies, by Product Segment
Direct Premiums, Life Insurance in Force	$ 979,755.7				$ 990,371.0				$ 979,755.7	$ 990,371.0	$ 937,300.8
Ceded Premiums, Life Insurance in Force	41,550.9				41,669.8				41,550.9	41,669.8	40,902.8
Assumed Premiums, Life Insurance in Force	896.4				1,018.4				896.4	1,018.4	1,089.3
Premiums, Net, Life Insurance in Force	$ 939,101.2				$ 949,719.6				$ 939,101.2	$ 949,719.6	$ 897,487.3
Life Insurance in Force Premiums, Percentage Assumed to Net	0.10%				0.10%				0.10%	0.10%	0.10%
Gross Amount									$ 9,621.9	$ 9,576.3	$ 9,171.1
Reinsurance Ceded									337.9	327.2	327.6
Assumed Premiums Earned									94.1	116.5	142.6
Net Amount	$ 2,319.9	$ 2,318.1	$ 2,368.7	$ 2,371.4	$ 2,352.6	$ 2,331.2	$ 2,343.1	$ 2,338.7	$ 9,378.1	$ 9,365.6	$ 8,986.1
Percentage of Amount Assumed to Net									1.00%	1.20%	1.60%
Life Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 2,536.8	$ 2,549.7	$ 2,442.1
Reinsurance Ceded									141.9	137.2	138.2
Assumed Premiums Earned									8.0	8.2	8.7
Net Amount									$ 2,402.9	$ 2,420.7	$ 2,312.6
Percentage of Amount Assumed to Net									0.30%	0.30%	0.40%
Accident, Health, and Other Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount									$ 7,085.1	$ 7,026.6	$ 6,729.0
Reinsurance Ceded									196.0	190.0	189.4
Assumed Premiums Earned									86.1	108.3	133.9
Net Amount									$ 6,975.2	$ 6,944.9	$ 6,673.5
Percentage of Amount Assumed to Net									1.20%	1.60%	2.00%