Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024 USD ($) Integer	Jun. 30, 2023 USD ($) Integer	Jun. 30, 2024 USD ($) Integer	Jun. 30, 2023 USD ($) Integer	Dec. 31, 2023 USD ($)
Schedule of Investments
Other Long-term Investments	$ 1,646,300,000		$ 1,646,300,000		$ 1,579,400,000
Accrued Investment Income	713,900,000		713,900,000		633,900,000
Write-offs			0		0
Gains (Losses) on Sales on Loan Foreclosure	0	$ 0	0	$ 0
Commitments to Fund Commercial Mortgage Loans	0		0
Real Estate Investment Property, Net	62,000,000.0		62,000,000.0		64,400,000
Real Estate Held-for-sale	$ 40,900,000		$ 40,900,000		40,900,000
Repurchase agreements - Typical Days outstanding			30
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral	102.00%		102.00%
Gross Securities Loaned	$ 57,500,000		$ 57,500,000		72,000,000.0
Outstanding Repurchase Agreements	0		0		0
Off Balance Sheet Amount
Schedule of Investments
Securities Received as Collateral	38,600,000		38,600,000		12,500,000
Cash Collateral for Borrowed Securities	22,700,000		22,700,000		63,100,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	2,273,200,000		2,273,200,000		2,328,400,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	12,100,000		12,100,000		10,200,000
Accrued Investment Income	7,100,000		7,100,000		7,200,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0		0		0
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	$ 0	$ 0	$ 0	$ 0
Financing Receivable, Modifications, Number of Contracts | Integer	0	0	0	0
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments	$ 0		$ 0
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	1,407,200,000		1,407,200,000		1,326,500,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability	86,000,000.0		86,000,000.0
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	300,000		300,000		300,000
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	$ 1,406,900,000		$ 1,406,900,000		$ 1,326,200,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	929		929
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	834		834
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	74		74
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	67		67