Schedule IV Reinsurance (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reinsurance Premiums for Insurance Companies, by Product Segment
Direct Premiums, Life Insurance in Force	$ 1,079,042.5	$ 1,057,312.2	$ 1,007,562.2
Ceded Premiums, Life Insurance in Force	21,210.7	35,773.9	37,794.6
Assumed Premiums, Life Insurance in Force	921.6	931.9	920.3
Premiums, Net, Life Insurance in Force	$ 1,058,753.4	$ 1,022,470.2	$ 970,687.9
Life Insurance in Force Premiums, Percentage Assumed to Net	0.10%	0.10%	0.10%
Gross Amount	$ 10,286.8	$ 9,893.6	$ 9,736.8
Reinsurance Ceded	321.2	355.8	352.4
Assumed Premiums Earned	80.4	78.7	90.6
Net Amount	$ 10,046.0	$ 9,616.5	$ 9,475.0
Percentage of Amount Assumed to Net	0.80%	0.80%	1.00%
Life Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount	$ 2,799.0	$ 2,704.3	$ 2,544.7
Reinsurance Ceded	157.5	155.1	150.2
Assumed Premiums Earned	6.0	6.5	7.6
Net Amount	$ 2,647.5	$ 2,555.7	$ 2,402.1
Percentage of Amount Assumed to Net	0.20%	0.30%	0.30%
Accident, Health, and Other Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment
Gross Amount	$ 7,487.8	$ 7,189.3	$ 7,192.1
Reinsurance Ceded	163.7	200.7	202.2
Assumed Premiums Earned	74.4	72.2	83.0
Net Amount	$ 7,398.5	$ 7,060.8	$ 7,072.9
Percentage of Amount Assumed to Net	1.00%	1.00%	1.20%