Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2022 USD ($) Integer $ / shares	Dec. 31, 2021 USD ($) $ / shares	Dec. 31, 2020 USD ($) $ / shares	Jan. 01, 2021 USD ($)
New Accounting Pronouncements or Change in Accounting Principle
Number of Operating Segments | Integer	3
Premium Income	$ 9,623,400,000	$ 9,481,000,000	$ 9,378,100,000
Commissions	1,086,400,000	1,038,100,000	1,057,300,000
Ceded Policy Loans	3,312,500,000	3,373,700,000
Premium Receivable, Allowance for Credit Loss	32,500,000	34,200,000
Premiums Receivable, Gross	557,600,000	530,700,000
Premium Receivable, Credit Loss Expense (Reversal)	(1,700,000)	(4,600,000)
Accumulated Depreciation for Property and Equipment	1,328,700,000	1,259,600,000
Value of Business Acquired	62,900,000	73,100,000
Accumulated Amortization of Value of Business Acquired	150,300,000	157,700,000
Amortization of Value of Business Acquired	4,900,000	5,700,000	6,100,000
Prepaid Reinsurance Premiums	713,300,000	777,100,000
Deferred Gain on Reinsurance	3,100,000	3,700,000
Reinsurance Recoverable, Allowance for Credit Loss	1,700,000	2,300,000
Reinsurance Recoverable, Credit Loss Expense (Reversal)	(600,000)	(9,400,000)
Premium Tax Expense	169,300,000	166,000,000	175,500,000
Participation Fund Account Assets (PFA)	250,200,000	297,700,000
Net Income	$ 1,314,200,000	$ 824,200,000	$ 793,000,000.0
Diluted Earnings Per Share | $ / shares	$ 6.50	$ 4.02	$ 3.89
Accumulated Other Comprehensive Income (Loss)	$ (2,756,600,000)	$ 354,100,000
Basic Earnings Per Share | $ / shares	$ 6.55	$ 4.04	$ 3.89
Pro Forma | Accounting Standards Update 2018-12 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Accumulated Other Comprehensive Income (Loss)				$ 6,700,000,000
Minimum | Pro Forma | Accounting Standards Update 2018-12 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Net Income	$ 80,000,000	$ 145,000,000
Diluted Earnings Per Share | $ / shares	$ 0.40	$ 0.70
Accumulated Other Comprehensive Income (Loss)	$ 600,000,000	$ 5,400,000,000
Maximum | Pro Forma | Accounting Standards Update 2018-12 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Net Income	$ 110,000,000	$ 175,000,000
Diluted Earnings Per Share | $ / shares	$ 0.55	$ 0.85
Accumulated Other Comprehensive Income (Loss)	$ 800,000,000	$ 5,600,000,000
Change in Accounting Method Accounted for as Change in Estimate
New Accounting Pronouncements or Change in Accounting Principle
Premium Income	(13,400,000)
Commissions	(1,000,000.0)
Net Income	$ (9,800,000)
Diluted Earnings Per Share | $ / shares	$ 0.05
Basic Earnings Per Share | $ / shares	$ 0.05