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Stock-Based Compensation - Restricted Stock Unit Activity (Details) - Restricted Stock Units (RSUs)
shares in Thousands
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Numbers of Units Outstanding  
Awarded and unvested, beginning of period (in shares) 2,834
Restricted stock units (in shares) 821
Vested (in shares) (954)
Forfeited (in shares) (408)
Awarded and unvested, ending period (in shares) 2,293
Restricted stock units expected to vest, ending period (in shares) 1,954
Weighted Average Grant Date Fair Value  
Awarded and unvested, beginning period (in dollars per share) | $ / shares $ 43.52
Granted (in dollars per share) | $ / shares 47.21
Vested (in dollars per share) | $ / shares 43.91
Forfeited (in dollars per share) | $ / shares 43.78
Awarded and unvested, ending period (in dollars per share) | $ / shares $ 44.64