Investments - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Schedule of Investments
Write-offs	$ 0.0	$ 0.0
Other Long-term Investments	1,619.8		$ 1,579.4
Accrued Investment Income	660.9		633.9
Loan Foreclosure Expense	0.0	0.0
Commitments to Fund Commercial Mortgage Loans	1.8
Real Estate Investment Property, Net	63.2		64.4
Real Estate Held-for-sale	$ 40.9		40.9
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	$ 60.2		72.0
Outstanding Repurchase Agreements	0.0		0.0
Equity Securities
Schedule of Investments
Unrealized Gain (Loss) on Investments	(0.1)	0.7
Private Equity Partnerships
Schedule of Investments
Unrealized Gain (Loss) on Investments	25.0	17.8
Management Fee Expense	(4.7)	(3.5)
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	46.9		63.1
Securities Received as Collateral	16.1		12.5
Commercial Real Estate
Schedule of Investments
Write-offs	0.0	0.0
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	2,309.5		2,328.4
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	10.7	$ 10.0	10.2	$ 9.3
Accrued Investment Income	$ 7.2		7.2
Financing Receivable, Modifications, Number of Contracts	0.0	0.0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	$ 0.0		0.0
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	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	1,370.6		1,326.5
Private Placement Fixed Maturity Security [Member]
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability	74.5
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	0.3		0.3
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	$ 1,370.3		$ 1,326.2
External Credit Rating, Investment Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	903
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions	826
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	75
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions	70