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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, 2017
$ / shares
shares
Shares  
Nonvested, beginning of period (in shares) | shares 180,656
Granted (in shares) | shares 84,528
Vested (in shares) | shares (32,675)
Forfeited (in shares) | shares (9,171)
Nonvested, end of period (in shares) | shares 223,338
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period (in dollars per share) | $ / shares $ 57.99
Granted (in dollars per share) | $ / shares 97.49
Vested (in dollars per share) | $ / shares 63.45
Forfeited (in dollars per share) | $ / shares 60.17
Nonvested, end of period (in dollars per share) | $ / shares $ 71.95