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Investments (Available For Sale Securities by Contractual Maturity) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Debt Securities [Member]    
Fixed maturities, available for sale    
Amortized Cost $ 2,505,953 $ 2,586,821
Estimated Fair Value 2,546,107 2,613,406
U.S. Treasury obligations [Member]    
Fixed maturities, available for sale    
Amortized Cost 143,088 146,186
Due in one year or less 29,357  
Due after one year through five years 92,270  
Due after five years through ten years 18,815  
Due after ten years 3,011  
Estimated Fair Value 143,453 146,539
U.S. Government-sponsored enterprise obligations [Member]    
Fixed maturities, available for sale    
Amortized Cost 20,179 30,038
Due in one year or less 499  
Due after one year through five years 7,873  
Due after five years through ten years 11,678  
Due after ten years 142  
Estimated Fair Value 20,192 30,235
State and municipal bonds [Member]    
Fixed maturities, available for sale    
Amortized Cost 702,688 790,154
Due in one year or less 54,052  
Due after one year through five years 243,955  
Due after five years through ten years 308,236  
Due after ten years 115,593  
Estimated Fair Value 721,836 800,463
Corporate debt [Member]    
Fixed maturities, available for sale    
Amortized Cost 1,294,689 1,264,812
Due in one year or less 138,176  
Due after one year through five years 733,558  
Due after five years through ten years 418,955  
Due after ten years 22,039  
Estimated Fair Value 1,312,728 1,278,991
Residential mortgage-backed securities [Member]    
Fixed maturities, available for sale    
Amortized Cost 204,179 216,285
Estimated Fair Value 206,415 217,906
Agency commercial mortgage-backed securities [Member]    
Fixed maturities, available for sale    
Amortized Cost 11,945 12,837
Estimated Fair Value 11,960 12,783
Other commercial mortgage-backed securities [Member]    
Fixed maturities, available for sale    
Amortized Cost 17,087 19,571
Estimated Fair Value 17,157 19,611
Other asset-backed securities [Member]    
Fixed maturities, available for sale    
Amortized Cost 112,098 106,938
Estimated Fair Value $ 112,366 $ 106,878