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Employment Expenses and Benefit Plans (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Retirement Benefits [Abstract]      
Employer matching contribution, percent 100.00% 100.00% 100.00%
401(k) percent of match 4.00% 4.00% 4.00%
Annual contribution $ 11,500 $ 10,800 $ 9,900
Stock purchase plan, percent of match 15.00%    
Stock purchase plans compensation expense $ 860 $ 826 $ 835