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Stock-Based Compensation - Schedule of Restricted Stock Awards (Details) - Restricted Stock Awards - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Summary of restricted stock award activity (in shares)      
Outstanding at beginning of period (in shares) 43,175 49,825 49,869
Granted (in shares) 158,500 0 5,900
Vested (in shares) (40,225) (6,650) (5,944)
Forfeited (in shares) 0 0 0
Outstanding at end of period (in shares) 161,450 43,175 49,825
Weighted-Average Grant Date Fair Value per Share (in usd per share)      
Outstanding at beginning of period (in USD per share) $ 33.87 $ 33.76 $ 33.40
Granted (in USD per share) 28.05 0.00 25.44
Vested (in USD per share) 34.49 33.02 22.55
Forfeited (in USD per share) 0.00 0.00 0.00
Outstanding at end of period (in USD per share) $ 28.00 $ 33.87 $ 33.76
Additional disclosures      
Aggregate intrinsic value $ 4.6    
Weighted-average remaining contractual term of non-vested shares 1 year 4 months 18 days