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INVESTMENTS (Schedule of Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Available-for-sale, Amortized Cost    
Due in one year or less $ 10,124  
Due after one year through five years 116,798  
Due after five years through ten years 195,301  
Due after ten years 506,326  
Amortized cost, total 828,549  
Available-for-sale, Fair Value    
Due in one year or less 10,080  
Due after one year through five years 113,751  
Due after five years through ten years 187,899  
Due after ten years 487,270  
Fair value, total 799,000  
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Due in one year or less 2,575  
Due after one year through five years 3,598  
Due after five years through ten years 17,022  
Due after ten years 69,867  
Amortized Cost 93,062 $ 94,073
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value [Abstract]    
Due in one year or less 2,583  
Due after one year through five years 3,626  
Due after five years through ten years 16,924  
Due after ten years 68,468  
Held-to-maturity Securities, Fair Value $ 91,601 $ 94,913