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Investments - Additional Information (Detail)
12 Months Ended
Dec. 31, 2018
USD ($)
Integer
Dec. 31, 2017
USD ($)
Integer
Dec. 31, 2016
USD ($)
Integer
Schedule of Investments      
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss $ 0.0 $ 0  
Commitment to Fund Private Placement Fixed Maturity Securities 117,300,000    
Carrying Amount of Variable Interest Entity Investments 575,300,000 535,400,000  
Unfunded Commitments 2,300,000 3,700,000  
Fixed Maturity Securities 43,011,700,000 45,457,800,000  
Other Long-term Investments $ 702,900,000 646,800,000  
Mortgage Loan Policy is not to Exceed a Loan-to-value Ratio, Percent 75.00%    
Loan to Value Ratio Update Frequency three    
Mortgage Loan Inspection Frequency two    
Mortgage 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    
Financing Receivable, Allowance for Credit Losses     $ 0
Foreclosed Real Estate Expense $ 0 0 0
Average Recorded Investment of Impaired Mortgage Loans 2,300,000 0 2,200,000
Interest Income on Mortgage Loans Subsequent to Impairment 0 0 0
Committments to Fund Commercial Mortgage Loans $ 13,000,000    
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 $ 164,100,000 159,200,000  
Outstanding Repurchase Agreements 0 0  
Off Balance Sheet Amount      
Schedule of Investments      
Cash Collateral for Borrowed Securities 100,000 30,500,000  
Securities Received as Collateral 171,400,000 135,600,000  
Commercial Real Estate Portfolio Segment      
Schedule of Investments      
Loans and Leases Receivable, Allowance 200,000 0 0
Provision for Loan and Lease Losses 200,000 0 $ 0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing 0 0  
Fixed Maturity Securities      
Schedule of Investments      
Fixed Maturity Securities $ 43,011,700,000 $ 45,457,800,000  
Commercial Real Estate Portfolio Segment      
Schedule of Investments      
Financing Receivable, Modifications, Subsequent Default, Number of Contracts | Integer     1
Financing Receivable, Modifications, Number of Contracts | Integer 1 0  
Impaired Financing Receivable, with No Related Allowance, Recorded Investment     $ 5,400,000
Financing Receivable, Modifications, Pre-Modification Recorded Investment $ 3,600,000    
Impaired Financing Receivable, with Related Allowance, Recorded Investment 3,400,000    
Special Purpose Entity | Bonds      
Schedule of Investments      
Fixed Maturity Securities 156,700,000 $ 154,100,000  
Special Purpose Entity | Partnership Interest      
Schedule of Investments      
Other Long-term Investments     $ 1,000,000
Gain (Loss) on Investments, Excluding Other than Temporary Impairments   (200,000)  
Partnership Interests In Tax Credit Investments      
Schedule of Investments      
Carrying Amount of Variable Interest Entity Investments 91,500,000 128,200,000  
Equity Method Investments      
Schedule of Investments      
Carrying Amount of Variable Interest Entity Investments $ 483,800,000 $ 407,200,000  
External Credit Rating, Investment Grade      
Schedule of Investments      
Number of Fixed Maturity Securities in an Unrealized Loss Position 552    
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year 204    
Fixed Maturity Securities $ 39,973,100,000    
External Credit Rating, Below-Investment-Grade      
Schedule of Investments      
Number of Fixed Maturity Securities in an Unrealized Loss Position 135    
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year 32    
Fixed Maturity Securities $ 3,038,600,000