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Stock-Based Compensation - Restricted Stock Award Activity (Details) - Restricted Stock Awards - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shares      
Nonvested at beginning of period (in shares) 261,296 264,521 238,669
Granted (in shares) 0 121,039 121,320
Vested (in shares) (113,479) (112,516) (92,486)
Forfeited (in shares) (10,418) (11,748) (2,982)
Nonvested at end of period (in shares) 137,399 261,296 264,521
Weighted-Average Grant Date Fair Value      
Nonvested at beginning of period (in usd per share) $ 16.80 $ 15.92 $ 15.20
Granted (in usd per share) 0.00 17.60 16.72
Vested (in usd per share) 16.55 15.62 15.12
Forfeited (in usd per share) 16.80 16.62 15.73
Nonvested at end of period (in usd per share) $ 17.00 $ 16.80 $ 15.92