Investments - Additional Information (Detail)		3 Months Ended		6 Months Ended
		Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)
Schedule of Investments
Other Long-term Investments		$ 1,659,900,000		$ 1,659,900,000		$ 1,694,400,000
Accrued Investment Income		702,900,000		$ 702,900,000		649,800,000
Financing Receivable, Modified in Period, Amount		$ 0	$ 0		$ 0
Financing Receivable, Modified, Weighted Average Term Increase from Modification		24 months		24 months
Gains (Losses) on Sales on Loan Foreclosure		$ 0	$ 0	$ 0	$ 0
Commitments to Fund Commercial Mortgage Loans		0		0
Real Estate Investment Property, Net		43,900,000		43,900,000		59,500,000
Real Estate Held-for-sale		$ 51,400,000		$ 51,400,000		41,900,000
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		$ 65,400,000		$ 65,400,000		94,000,000.0
Outstanding Repurchase Agreements		0		$ 0		0
Financing Receivable, Modified in Period, to Total Financing Receivables, Percentage				0.60%
Off Balance Sheet Amount
Schedule of Investments
Securities Received as Collateral		41,200,000		$ 41,200,000		34,800,000
Cash Collateral for Borrowed Securities		27,100,000		27,100,000		62,700,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	[1]	2,188,100,000		2,188,100,000		2,240,600,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	15,600,000		15,600,000		16,100,000
Accrued Investment Income		6,700,000		6,700,000
Financing Receivable, Modified in Period, Amount		14,200,000		14,200,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		1,000,000		1,000,000		1,000,000
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments		0		0		100,000
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		1,451,100,000		1,451,100,000		1,450,800,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability		98,100,000		98,100,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		200,000		200,000		200,000
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		$ 1,450,900,000		$ 1,450,900,000		$ 1,450,600,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position		867		867
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		756		756
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position		55		55
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		40		40

[1] 1Loan-to-Value Ratio utilizes the most recent internal valuation of the property