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Stock Options, Restricted Stock and Restricted Stock Units - RSU (Details) - Restricted stock and restricted stock units [Member] - $ / shares
1 Months Ended 12 Months Ended
May 31, 2017
Feb. 28, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of restricted stock and restricted stock units activity          
Beginning Balance, Number of shares or units     727,100 634,327 524,216
Grants, Number of shares or units 7,541 67,599 75,140 188,737 126,655
Vested shares, Number of shares or units     (84,908) (23,539) (13,725)
Forfeitures, Number of shares or units     (1,204) (72,425) (2,819)
Ending Balance, Number of shares or units     716,128 727,100 634,327
Beginning Balance, Weighted-average grant price     $ 97.53 $ 104.66 $ 115.67
Grants, Weighted-average grant price     82.18 50.63 61.00
Vested shares, Weighted-average grant price     66.60 50.43 52.94
Forfeitures, Weighted-average grant price     62.28 62.41 115.55
Ending Balance, Weighted-average grant price     $ 99.65 $ 97.53 $ 104.66