Investments - Additional Information (Detail)		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)	Jul. 01, 2025 USD ($)
Schedule of Investments
Other Long-term Investments		$ 1,682,200,000		$ 1,682,200,000		$ 1,694,400,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans		15,600,000		15,600,000		16,100,000
Accrued Investment Income		735,700,000		$ 735,700,000		649,800,000
Financing Receivable, Modified in Period, Amount		0	$ 0		$ 0
Financing Receivable, Modified, Weighted Average Term Increase from Modification				24 months
Gains (Losses) on Sales on Loan Foreclosure				$ 0		0
Commitments to Fund Commercial Mortgage Loans		9,100,000		9,100,000
Real Estate Investment Property, Net		42,800,000		42,800,000		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		$ 129,800,000		$ 129,800,000		94,000,000.0
Outstanding Repurchase Agreements		0		$ 0		0
Financing Receivable, Modified in Period, to Total Financing Receivables, Percentage				0.70%
Net Investment Loss		128,000,000.0	(12,900,000)	$ (96,500,000)	(24,500,000)
Mortgage Loans (net of allowance for credit losses of $15.6; $16.1)	[1],[2]	2,129,800,000		2,129,800,000		2,224,500,000
Financial Asset, Past Due
Schedule of Investments
Mortgage Loans (net of allowance for credit losses of $15.6; $16.1)		8,400,000		8,400,000		9,200,000
Operating Segments
Schedule of Investments
Net Investment Loss		128,000,000.0	(12,900,000)	(96,500,000)	(24,500,000)
Long-term Care and Individual Disability Reinsurance Transaction
Schedule of Investments
Accrued Investment Income							$ 47,100,000
Long-term Care and Individual Disability Reinsurance Transaction | Operating Segments
Schedule of Investments
Net Investment Loss		137,600,000	$ 0	(46,800,000)	$ 0
Subsidiary Dividend Funding
Schedule of Investments
Net Investment Loss				19,100,000
Off Balance Sheet Amount
Schedule of Investments
Securities Received as Collateral		26,000,000.0		26,000,000.0		34,800,000
Cash Collateral for Borrowed Securities		109,000,000.0		109,000,000.0		62,700,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	[1]	2,145,400,000		2,145,400,000		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		6,800,000		6,800,000		7,000,000.0
Write-offs		800,000
Financing Receivable, Modified in Period, Amount				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						100,000
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		1,456,500,000		1,456,500,000		1,450,800,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability		61,400,000		61,400,000
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		$ 1,456,300,000		$ 1,456,300,000		$ 1,450,600,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position		825		825
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		765		765
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position		47		47
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		33		33

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

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