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Investments - Additional Information (Detail)
3 Months Ended
Mar. 31, 2020
USD ($)
Integer
Mar. 31, 2019
USD ($)
Integer
Jan. 01, 2020
USD ($)
Dec. 31, 2019
USD ($)
Schedule of Investments        
Allowance for credit losses for Fixed Maturity Securities $ 48,000,000.0     $ 0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not to Sell before Recovery, Credit Loss, Previously Recorded, Expense (Reversal) 48,000,000.0      
Debt Securities, Available-for-sale, Allowance for Credit Loss, Sell before Recovery 5,900,000      
Commitment to Fund Private Placement Fixed Maturity Securities 88,800,000      
Carrying Amount of Variable Interest Entity Investments 689,200,000     675,100,000
Unfunded Commitments 1,900,000      
Accrued Investment Income 748,500,000     693,000,000.0
Loan Foreclosure Expense 0 $ 0    
Allowance for Loan and Lease Losses, Write-offs   300,000    
Provision for Loan and Lease Losses   $ 100,000    
Commitments to Fund Commercial Mortgage Loans $ 17,500,000      
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 Borrowed $ 4,900,000     176,400,000
Outstanding Repurchase Agreements 0     0
Off Balance Sheet Amount        
Schedule of Investments        
Securities Received as Collateral 5,500,000     186,500,000
Commercial Real Estate        
Schedule of Investments        
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans 2,463,200,000     2,397,000,000.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans 11,100,000   $ 8,300,000  
Accrued Investment Income $ 8,500,000     8,300,000
Financing Receivable, Modifications, Number of Contracts | Integer 0 0    
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing $ 0     0
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment 0 $ 2,300,000    
Unfunded Mortgage Loan Commitments        
Schedule of Investments        
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments 100,000   $ 300,000  
All Other Corporate Bonds        
Schedule of Investments        
Allowance for credit losses for Fixed Maturity Securities 48,000,000.0      
Partnership Interests In Tax Credit Investments        
Schedule of Investments        
Carrying Amount of Variable Interest Entity Investments 51,100,000     58,400,000
Equity Method Investments        
Schedule of Investments        
Carrying Amount of Variable Interest Entity Investments $ 638,100,000     $ 616,700,000
External Credit Rating, Investment Grade        
Schedule of Investments        
Number of Fixed Maturity Securities in an Unrealized Loss Position 336      
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year 8      
External Credit Rating, Below-Investment-Grade        
Schedule of Investments        
Number of Fixed Maturity Securities in an Unrealized Loss Position 127      
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year 14