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Employee Benefit Plans (Summary of the Activity in the SERP's Projected Benefit Obligation) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Benefit obligation:      
Balance, January 1 $ 85,551 $ 78,369  
Service cost 3,733 3,559  
Interest cost 3,621 3,505  
Benefits paid (2,031) (1,885)  
Actuarial (gain) loss 7,916 2,003  
Balance, December 31 98,790 85,551 $ 78,369
Supplemental Executive Retirement Plan [Member]      
Benefit obligation:      
Balance, January 1 9,132 8,748  
Service cost 143 133 140
Interest cost 360 367 348
Benefits paid (247) (247) (243)
Actuarial (gain) loss 706 131  
Balance, December 31 $ 10,094 $ 9,132 $ 8,748