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Share-Based Compensation - Information for Restricted Stock Units under Equity Incentive Plan (Details) - Restricted stock units
12 Months Ended
Dec. 31, 2018
$ / shares
shares
Shares  
Nonvested, beginning of period (in shares) | shares 637,667
Granted (in shares) | shares 223,918
Vested (in shares) | shares (219,305)
Forfeited (in shares) | shares (44,984)
Nonvested, end of period (in shares) | shares 597,296
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period (usd per share) | $ / shares $ 135.86
Granted (usd per share) | $ / shares 294.50
Vested (usd per share) | $ / shares 130.72
Forfeited (usd per share) | $ / shares 172.32
Nonvested, end of period (usd per share) | $ / shares $ 194.48