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Earnings Per Share (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Numerator [Abstract]      
Net income $ 51,493 $ 47,346 $ 46,432
Basic earnings per share [Abstract]      
Weighted average shares (in shares) 17,631,000 17,548,000 17,431,000
Diluted earnings per share [Abstract]      
Dilutive effect of equity grants (in shares) 22,000 18,000 52,000
Adjusted weighted average shares (in shares) 17,653,000 17,566,000 17,483,000
Earnings per share [Abstract]      
Basic earnings per share (in dollars per share) $ 2.92 $ 2.70 $ 2.66
Diluted earnings per share (in dollars per share) $ 2.92 $ 2.70 $ 2.66
Options [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Options excluded from diluted calculations (in shares) 0 0 58,063
Weighted average price of outstanding options (in dollars per share)     $ 35.409