Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) Integer $ / shares	Dec. 31, 2023 USD ($) $ / shares	Dec. 31, 2022 USD ($) $ / shares
New Accounting Pronouncements or Change in Accounting Principle
Number of Operating Segments | Integer	3
Premium Income	$ 10,497.4	$ 10,046.0	$ 9,616.5
Commissions	1,258.6	1,170.1	1,086.4
Ceded Policy Loans	3,313.6	3,322.5
Premium Receivable, Allowance for Credit Loss	26.8	29.5
Premiums Receivable, Gross	584.1	612.4
Premium Receivable, Credit Loss Expense (Reversal)	(2.7)	(3.0)
Accumulated Depreciation for Property and Equipment	1,501.3	1,422.8
Value of Business Acquired	55.1	63.9
Accumulated Amortization of Value of Business Acquired	165.5	161.8
Amortization of Value of Business Acquired	6.2	5.2	4.9
Prepaid Reinsurance Premiums	508.0	549.4
Deferred Gain on Reinsurance	4.9	8.8
Reinsurance Recoverable, Allowance for Credit Loss	1.5	1.7
Premium Tax Expense	194.5	183.5	$ 169.3
Participation Fund Account Assets (PFA)	$ 231.3	$ 244.4
Basic Earnings Per Share | $ / shares	$ 9.49	$ 6.53	$ 7.01
Diluted Earnings Per Share | $ / shares	$ 9.46	$ 6.50	$ 6.96
Net Income	$ 1,779.1	$ 1,283.8	$ 1,407.2
Change in Accounting Method Accounted for as Change in Estimate
New Accounting Pronouncements or Change in Accounting Principle
Premium Income			(13.4)
Commissions			$ (1.0)
Basic Earnings Per Share | $ / shares			$ 0.05
Diluted Earnings Per Share | $ / shares			$ 0.05
Net Income			$ (9.8)