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Stock-based Compensation Expense Activity in Non-Performance Based Restricted Stock Awards (Details) - Restricted Stock And Restricted Stock Units
12 Months Ended
Dec. 31, 2018
$ / shares
shares
Shares  
Nonvested, beginning of period (in shares) | shares 223,338
Granted (in shares) | shares 104,513
Vested (in shares) | shares (48,178)
Forfeited (in shares) | shares (22,155)
Nonvested, end of period (in shares) | shares 257,518
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period (in dollars per share) | $ / shares $ 71.95
Granted (in dollars per share) | $ / shares 104.36
Vested (in dollars per share) | $ / shares 66.93
Forfeited (in dollars per share) | $ / shares 78.98
Nonvested, end of period (in dollars per share) | $ / shares $ 85.44