Investments - Additional Information (Detail)		12 Months Ended
		Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jul. 01, 2025 USD ($)
Schedule of Investments
Other Long-term Investments		$ 1,670,400,000	$ 1,694,400,000
Financing Receivable, Allowance for Credit Losses		15,900,000	16,100,000
Accrued Investment Income		$ 596,000,000.0	649,800,000
Financing Receivable, Modified, Weighted Average Term Increase from Modification		24 months
Foreclosed Real Estate Expense		$ 0	0	$ 0
Committments to Fund Commercial Mortgage Loans		0	17,900,000
Real Estate Investment Property, Net		41,700,000	59,500,000
Real Estate Held-for-sale		$ 41,900,000	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%
Gross Securities Loaned		$ 106,100,000	94,000,000.0
Outstanding Repurchase Agreements		0	0
Financing Receivable, Nonaccrual			1,000,000	1,000,000
Financing Receivable, Nonaccrual, Interest Income		0	0	0
Mortgage Loans	[1]	2,109,500,000	2,224,500,000
Other Investments Gross Realized Losses		2,100,000	0	1,000,000.0
Net Investment Loss		(106,600,000)	(34,600,000)	(36,000,000.0)
Long-term Care and Individual Disability Reinsurance Transaction
Schedule of Investments
Accrued Investment Income					$ 47,100,000
Operating Segments
Schedule of Investments
Net Investment Loss		(106,600,000)	(34,600,000)	(36,000,000.0)
Operating Segments | Long-term Care and Individual Disability Reinsurance Transaction
Schedule of Investments
Net Investment Loss		(46,800,000)	0	0
Financial Asset, Past Due
Schedule of Investments
Mortgage Loans		8,400,000	9,200,000
Private Equity Partnerships
Schedule of Investments
Unrealized Gain (Loss) on Investments		128,100,000	127,100,000	102,900,000
Management Fee Expense		36,900,000	24,000,000	24,800,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities		76,100,000	62,700,000
Securities Received as Collateral		34,100,000	34,800,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss		2,125,400,000	2,240,600,000
Financing Receivable, Allowance for Credit Losses		15,900,000	16,100,000	$ 10,200,000
Accrued Investment Income		6,700,000	7,000,000.0
Financing Receivable, Allowance for Credit Loss, Writeoff		800,000	7,000,000.0
Financing Receivable, Troubled Debt Restructuring, Postmodification		14,200,000
Other Investments Gross Realized Losses		1,100,000
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments		0	100,000
Private Equity Partnerships | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		1,456,300,000	$ 1,450,600,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability		$ 87,500,000
External Credit Rating, Investment Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions		822
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		767
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions		40
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		30

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