v3.25.1
Investments - Additional Information (Detail)
$ in Millions
3 Months Ended
Mar. 31, 2025
USD ($)
Mar. 31, 2024
USD ($)
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Schedule of Investments        
Other Long-term Investments $ 1,689.8   $ 1,694.4  
Financing Receivable, Allowance for Credit Losses, Mortgage Loans 16.6   16.1  
Accrued Investment Income 622.9   649.8  
Loan Foreclosure Expense 0.0 $ 0.0    
Commitments to Fund Commercial Mortgage Loans 8.0      
Real Estate Investment Property, Net 54.5   59.5  
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 $ 61.8   94.0  
Outstanding Repurchase Agreements 0.0   0.0  
Financing Receivable, Modified, Commitment to Lend 14.2      
Private Equity Partnerships        
Schedule of Investments        
Unrealized Gain (Loss) on Investments 26.4 25.0    
Management Fee Expense (8.1) (4.7)    
Off Balance Sheet Amount        
Schedule of Investments        
Cash Collateral for Borrowed Securities 45.2   62.7  
Securities Received as Collateral 19.1   34.8  
Commercial Real Estate        
Schedule of Investments        
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans [1] 2,204.3   2,240.6  
Financing Receivable, Allowance for Credit Losses, Mortgage Loans [1] 16.6 $ 10.7 16.1 $ 10.2
Accrued Investment Income 6.3   7.0  
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing 1.0   1.0  
Unfunded Mortgage Loan Commitments        
Schedule of Investments        
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments 0.1   0.0  
Variable Interest Entity, Not Primary Beneficiary        
Schedule of Investments        
Other Long-term Investments 1,435.5   1,450.8  
Private Placement Fixed Maturity Security        
Schedule of Investments        
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability 25.0      
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary        
Schedule of Investments        
Other Long-term Investments 0.2   0.2  
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary        
Schedule of Investments        
Other Long-term Investments $ 1,435.3   $ 1,450.6  
External Credit Rating, Investment Grade        
Schedule of Investments        
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions 885      
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions 767      
External Credit Rating, Below-Investment-Grade        
Schedule of Investments        
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions 79      
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions 43      
[1]
1Loan-to-Value Ratio utilizes the most recent internal valuation of the property