Investments - Additional Information (Detail) $ in Millions		3 Months Ended
		Mar. 31, 2026 USD ($)	Mar. 31, 2025 USD ($)	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Schedule of Investments
Other Long-term Investments		$ 1,664.8		$ 1,670.4
Financing Receivable, Allowance for Credit Losses, Mortgage Loans		15.3		15.9
Accrued Investment Income		582.0		596.0
Loan Foreclosure Expense		0.0	$ 0.0
Commitments to Fund Commercial Mortgage Loans		0.0
Real Estate Investment Property, Net		40.6		41.7
Real Estate Held-for-sale		$ 41.9		41.9
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		$ 95.7		106.1
Cash Collateral for Borrowed Securities		61.7		76.1
Securities Received as Collateral		37.6		34.1
Outstanding Repurchase Agreements		0.0		0.0
Financing Receivable, Modified, Weighted-Average Term Increase from Modification			24 months
Line of Credit | FABL Trust
Schedule of Investments
Line of Credit Facility, Fair Value of Amount Outstanding		0.0
Private Equity Partnerships
Schedule of Investments
Unrealized Gain (Loss) on Investments		29.7	$ 26.4
Management Fee Expense		(6.5)	(8.1)
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	[1]	2,081.6		2,125.4
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	15.3	$ 16.6	15.9	$ 16.1
Accrued Investment Income		6.6		6.7
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0.0		0.0
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments		0.0		0.0
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments				$ 1,456.3
Private Placement Fixed Maturity Security
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability		158.9
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		$ 1,444.1
External Credit Rating, Investment Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions		900
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions		753
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions		66
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions		27

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