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Stock-Based Compensation - Restricted Stock Awards Narrative (Details) - Restricted Stock Awards - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expense $ 1.4 $ 1.0 $ 1.0 $ 1.0
Fair value of restricted stock vested 0.7 0.9 1.0 0.8
Unrecognized compensation expense $ 4.6 $ 3.5 $ 4.7 $ 5.8
Weighted-average period the unrecognized compensation cost is expected to be recognized 3 years 14 days