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Stock-Based Compensation - Schedule of Restricted Units (Details) - Restricted Stock Units - 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) 348,538 425,438 604,214
Granted and fully vested (in shares) 59,025 69,302 51,292
Issued (in shares) 0 (146,202) (230,068)
Forfeited (in shares) 0 0 0
Outstanding at end of period (in shares) 407,563 348,538 425,438
Weighted-Average Grant Date Fair Value per Share (in usd per share)      
Outstanding at beginning of period (in USD per share) $ 15.01 $ 13.81 $ 12.48
Granted and fully vested (USD per share) 23.04 20.62 27.87
Issued (USD per share) 0.00 14.17 13.45
Forfeited (in USD per share) 0 0 0
Outstanding at end of period (in USD per share) $ 16.17 $ 15.01 $ 13.81
Additional disclosures      
Aggregate intrinsic value $ 11.7