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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, 2018
Dec. 31, 2017
Investment [Line Items]    
Within one year, Amortized Cost $ 76,875  
After one year through five years, Amortized Cost 256,614  
After five years through ten years, Amortized Cost 81,311  
After ten years, Amortized Cost 189,926  
Held-to-maturity, Amortized Cost, Totals 613,878 $ 629,668
Within one year, Fair Value 76,861  
After one year through five years, Fair Value 257,530  
After five years through ten years, Fair Value 81,755  
After ten years, Fair Value 191,213  
Held-to-maturity, Fair Value, Totals 616,772 639,309
U.S. Agency-Based Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Held-to-maturity, Amortized Cost 8,102  
Held-to-maturity, Amortized Cost, Totals 8,102 10,260
Held-to-maturity, Fair Value 8,349  
Held-to-maturity, Fair Value, Totals 8,349 10,845
Asset-Backed Securities [Member]    
Investment [Line Items]    
Held-to-maturity, Amortized Cost 1,050  
Held-to-maturity, Amortized Cost, Totals 1,050 1,299
Held-to-maturity, Fair Value 1,064  
Held-to-maturity, Fair Value, Totals $ 1,064 $ 1,309