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Stock-Based Compensation - Restricted Stock Units Activity (Details) - Restricted Stock Units
shares in Thousands
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Shares  
Outstanding at beginning of period (in shares) | shares 528
Granted (in shares) | shares 123
Vested (in shares) | shares (166)
Forfeited (in shares) | shares (56)
Outstanding at end of period (in shares) | shares 429
Weighted Average Fair Value  
Outstanding at beginning of the period (in dollars per share) | $ / shares $ 39.12
Granted (in dollars per share) | $ / shares 38.16
Vested (in dollars per share) | $ / shares 37.99
Forfeited (in dollars per share) | $ / shares 40.51
Outstanding at beginning of the period (in dollars per share) | $ / shares $ 39.10