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Share-Based Compensation - Restricted Stock Unit and Award Activity (Details)
6 Months Ended
Jun. 30, 2018
$ / shares
shares
Restricted Stock Units  
Number of Units/Awards  
Beginning balance (in shares) | shares 380,940
Granted (in shares) | shares 295,995
Converted into shares of common stock (in shares) | shares (63,154)
Forfeited/expired (in shares) | shares (10,520)
Ending balance (in shares) | shares 603,261
Weighted-average grant-date fair value  
Beginning balance (dollars per share) | $ / shares $ 41.29
Granted (dollars per share) | $ / shares 54.43
Converted into shares of common stock (dollars per share) | $ / shares 28.20
Forfeited/expired (dollars per share) | $ / shares 48.09
Ending balance (dollars per share) | $ / shares $ 48.99
Restricted Stock Awards  
Number of Units/Awards  
Beginning balance (in shares) | shares 83,228
Vested (in shares) | shares (41,784)
Forfeited/expired (in shares) | shares (253)
Ending balance (in shares) | shares 41,191
Weighted-average grant-date fair value  
Beginning balance (dollars per share) | $ / shares $ 46.23
Vested (dollars per share) | $ / shares 46.23
Forfeited/expired (dollars per share) | $ / shares 46.23
Ending balance (dollars per share) | $ / shares $ 46.23