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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
Dec. 31, 2015
Dec. 31, 2014
Investment [Line Items]    
Within one year, Amortized Cost Basis $ 112,187  
After one year through five years, Amortized Cost Basis 299,100  
After five years through ten years, Amortized Cost Basis 109,901  
After ten years, Amortized Cost Basis 70,970  
Held-to-Maturity, Amortized Cost 645,164 $ 639,631
Within one year, Fair Value 112,830  
After one year through five years, Fair Value 306,633  
After five years through ten years, Fair Value 114,362  
After ten years, Fair Value 73,882  
Total, Fair Value 662,276 $ 664,371
U.S. Agency-Based Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Held-to-maturity, Amortized Cost Basis 13,223  
Held-to-maturity, Fair Value 14,471  
Commercial Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Held-to-maturity, Amortized Cost Basis 37,494  
Held-to-maturity, Fair Value 37,683  
Asset-Backed Securities [Member]    
Investment [Line Items]    
Held-to-maturity, Amortized Cost Basis 2,289  
Held-to-maturity, Fair Value $ 2,415