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Investments - Additional Information (Detail)
3 Months Ended
Mar. 31, 2019
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
Schedule of Investments      
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities $ 0.0   $ 0
Impaired Financing Receivable, Average Recorded Investment 2,300,000 $ 1,100,000  
Commitment to Fund Private Placement Fixed Maturity Securities 138,500,000    
Carrying Amount of Variable Interest Entity Investments 577,300,000   575,300,000
Unfunded Commitments to Investment Partnerships 2,300,000    
Fixed Maturity Securities $ 44,782,600,000   43,011,700,000
Mortgage Loan Policy is not to Exceed a Loan-to-value Ratio, Percent 75.00%    
Loan to Value Ratio Update Frequency 3 years    
Mortgage Loan Inspection Frequency 2 years    
Mortage Loan Policy Debt Service Coverage Ratio Lower Range 125.00%    
Mortgage Loans Issuance, Term Years 25    
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans 0    
Impaired Financing Receivable, with No Related Allowance, Interest Income, Accrual Method $ 0 0  
Committments to Fund Commercial Mortgage Loans $ 12,800,000    
Repurchase agreements - Typical Days Outstanding 30 days    
Minimum Percent of Fair Value of Securities Loaned or Securities Purchased Under Repurchase Agreements to be Maintained as Collateral 102.00%    
Net Securities Borrowed Reported in Balance Sheet $ 137,600,000   164,100,000
Outstanding Repurchase Agreements 0   0
Off Balance Sheet Amount      
Schedule of Investments      
Cash Collateral for Borrowed Securities     100,000
Securities Received as Collateral 146,100,000   171,400,000
Commercial Real Estate Portfolio Segment      
Schedule of Investments      
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing 0   0
Provision for Loan and Lease Losses $ 100,000 $ 0  
Commercial Real Estate Portfolio Segment      
Schedule of Investments      
Number of Loan Modifications due to Loan Foreclosure 0 0  
Special Purpose Entity | Bonds      
Schedule of Investments      
Fixed Maturity Securities $ 157,900,000   156,700,000
Partnership Interests In Tax Credit Investments      
Schedule of Investments      
Carrying Amount of Variable Interest Entity Investments 82,600,000   91,500,000
Equity Method Investments      
Schedule of Investments      
Carrying Amount of Variable Interest Entity Investments $ 494,700,000   $ 483,800,000
External Credit Rating, Investment Grade      
Schedule of Investments      
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions 270    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions 178    
Fixed Maturity Securities $ 41,548,700,000    
External Credit Rating, Non Investment Grade      
Schedule of Investments      
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions 74    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions 56    
Fixed Maturity Securities $ 3,233,900,000