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Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Investment [Line Items]    
Within one year, Amortized Cost Basis $ 95,695  
After one year through five years, Amortized Cost Basis 302,699  
After five years through ten years, Amortized Cost Basis 107,854  
After ten years, Amortized Cost Basis 81,812  
Held-to-Maturity, Amortized Cost 601,314 $ 645,164
Within one year, Fair Value 96,339  
After one year through five years, Fair Value 311,060  
After five years through ten years, Fair Value 113,247  
After ten years, Fair Value 85,595  
Total, Fair Value 620,781 $ 662,276
U.S. Agency-Based Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Held-to-maturity, Amortized Cost Basis 290  
Held-to-maturity, Fair Value 290  
Commercial Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Held-to-maturity, Amortized Cost Basis 10,943  
Held-to-maturity, Fair Value 12,136  
Asset-Backed Securities [Member]    
Investment [Line Items]    
Held-to-maturity, Amortized Cost Basis 2,021  
Held-to-maturity, Fair Value $ 2,114