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Investments - Additional Information (Detail)
3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
Investment
MortgageLoan
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Investment
MortgageLoan
Schedule of Available-for-sale Securities [Line Items]      
Fixed maturity available-for-sale securities, at estimated fair value $ 1,294,321,000   $ 1,548,638,000
Components of OTTI reflected in the statements of income (loss) $ 0 $ 0  
FLORIDA      
Schedule of Available-for-sale Securities [Line Items]      
Concentration Of Commitment Collateral Percentage 28.00%    
UTAH      
Schedule of Available-for-sale Securities [Line Items]      
Concentration Of Commitment Collateral Percentage 27.00%    
MINNESOTA      
Schedule of Available-for-sale Securities [Line Items]      
Concentration Of Commitment Collateral Percentage 24.00%    
COLORADO      
Schedule of Available-for-sale Securities [Line Items]      
Concentration Of Commitment Collateral Percentage 11.00%    
NEW JERSEY      
Schedule of Available-for-sale Securities [Line Items]      
Concentration Of Commitment Collateral Percentage 10.00%    
Commercial Mortgage Loans that had Two or more Payments Delinquent      
Schedule of Available-for-sale Securities [Line Items]      
Mortgage loans on real estate, number of loans | MortgageLoan 0   0
Estimate of Fair Value, Fair Value Disclosure      
Schedule of Available-for-sale Securities [Line Items]      
Assets $ 618,842,000   $ 634,232,000
Estimate of Fair Value, Fair Value Disclosure | Mortgage loans on real estate      
Schedule of Available-for-sale Securities [Line Items]      
Total number of AFS securities in an unrealized loss position | Investment 121   79
Number of AFS investment securities | Investment 350   371
Prepayment premiums $ 482,000   $ 1,183,000
Assets [1] 21,560,000   26,049,000
Deferred net gains (losses) on derivatives recorded in AOCI that are expected to be reclassified during the next 12 months 5,211,000    
Deposited with insurance regulatory authorities      
Schedule of Available-for-sale Securities [Line Items]      
Investment securities, estimated fair value 6,469,000   4,246,000
Future      
Schedule of Available-for-sale Securities [Line Items]      
Collateral, fair value 2,373,000   2,488,000
BBB      
Schedule of Available-for-sale Securities [Line Items]      
Fixed maturity available-for-sale securities, at estimated fair value $ 105,274,000   $ 120,663,000
[1] The fair value of mortgage loans on real estate is estimated by discounting the future cash flows using the current rates at which similar loans would be made to borrowers with similar credit ratings and/or similar remaining maturities.