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Share-based Payments Share-based Payments (Details Textual)
9 Months Ended
Sep. 30, 2018
$ / shares
shares
Performance stock units settled in stock  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Percentage of earnings of the target number of units granted based on actual stock performance (Minimum) 0.00%
Percentage of earnings of the target number of units granted based on actual stock performance (Maximum) 200.00%
Number of shares granted 67,000
Weighted average grant date fair value | $ / shares $ 317.09
Performance stock units settled in cash  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Percentage of earnings of the target number of units granted based on actual stock performance (Minimum) 0.00%
Percentage of earnings of the target number of units granted based on actual stock performance (Maximum) 200.00%
Number of shares granted 45,000
Number of days for calculation of average closing stock price 30 days