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Investments - Additional Information (Detail)
$ in Millions
3 Months Ended
Mar. 31, 2022
USD ($)
Mar. 31, 2021
USD ($)
Dec. 31, 2021
USD ($)
Schedule of Investments      
Commitment to Fund Private Placement Fixed Maturity Securities $ 44.0    
Other Long-term Investments 1,280.4   $ 1,203.0
Unfunded Commitments 0.7    
Financing Receivable, Allowance for Credit Losses, Mortgage Loans 7.9   8.3
Accrued Investment Income 658.5   602.7
Loan Foreclosure Expense 0.0 $ 0.0  
Commitments to Fund Commercial Mortgage Loans 26.5    
Real Estate Held-for-sale $ 40.0   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 Borrowed $ 240.7   283.7
Outstanding Repurchase Agreements 0.0   0.0
Off Balance Sheet Amount      
Schedule of Investments      
Cash Collateral for Borrowed Securities 73.3   94.8
Securities Received as Collateral 176.0   198.6
Commercial Real Estate      
Schedule of Investments      
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans 2,537.4   2,568.7
Financing Receivable, Allowance for Credit Losses, Mortgage Loans 7.9   8.3
Accrued Investment Income $ 8.0   8.1
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     0.1
Variable Interest Entity, Not Primary Beneficiary      
Schedule of Investments      
Other Long-term Investments 1,053.1   987.9
Public Utilities      
Schedule of Investments      
Debt Securities, Available-for-sale, Credit losses on securities for which credit losses were not previously recorded 4.1    
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary      
Schedule of Investments      
Other Long-term Investments 7.2   9.3
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary      
Schedule of Investments      
Other Long-term Investments $ 1,045.9   $ 978.6
External Credit Rating, Investment Grade      
Schedule of Investments      
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions 563    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions 48    
External Credit Rating, Below-Investment-Grade      
Schedule of Investments      
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions 77    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions 8