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Incentive Plans (Summary of Additional Stock Option Information) (Details)
 12 Months Ended
Dec. 31, 2015
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Options outstanding | shares 465,756
Options outstanding, average remaining life (in years) 4 years 6 months 18 days
Options outstanding, aggregate intrinsic value | $ $ 9,311,711
Options exercisable | shares 127,077
Options exercisable, average remaining life (in years) 3 years 8 months 23 days
Options exercisable, aggregate intrinsic value | $ $ 3,053,997
$32.53 to $37.06 [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Lower range limit of exercise price (in dollars per share) | $ / shares $ 32.53
Upper range limit of exercise price (in dollars per share) | $ / shares $ 37.06
Options outstanding | shares 171,592
Options outstanding, average remaining life (in years) 3 years 10 months 20 days
Options outstanding, aggregate intrinsic value | $ $ 4,614,797
Options exercisable | shares 62,213
Options exercisable, average remaining life (in years) 3 years 6 months 12 days
Options exercisable, aggregate intrinsic value | $ $ 1,707,361
$37.07 to $39.45 [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Lower range limit of exercise price (in dollars per share) | $ / shares $ 37.07
Upper range limit of exercise price (in dollars per share) | $ / shares $ 39.45
Options outstanding | shares 5,615
Options outstanding, average remaining life (in years) 2 years 2 days
Options outstanding, aggregate intrinsic value | $ $ 132,891
Options exercisable | shares 5,615
Options exercisable, average remaining life (in years) 2 years 2 days
Options exercisable, aggregate intrinsic value | $ $ 132,891
$39.46 to $40.25 [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Lower range limit of exercise price (in dollars per share) | $ / shares $ 39.46
Upper range limit of exercise price (in dollars per share) | $ / shares $ 40.25
Options outstanding | shares 218,899
Options outstanding, average remaining life (in years) 4 years 7 months 14 days
Options outstanding, aggregate intrinsic value | $ $ 4,564,023
Options exercisable | shares 59,249
Options exercisable, average remaining life (in years) 4 years 1 month 5 days
Options exercisable, aggregate intrinsic value | $ $ 1,213,745
$40.26 to $70.57 [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Lower range limit of exercise price (in dollars per share) | $ / shares $ 40.26
Upper range limit of exercise price (in dollars per share) | $ / shares $ 70.57
Options outstanding | shares 69,650
Options outstanding, average remaining life (in years) 6 years 1 month 13 days
Options outstanding, aggregate intrinsic value | $ $ 0
Options exercisable | shares 0
Options exercisable, average remaining life (in years) 0 years
Options exercisable, aggregate intrinsic value | $ $ 0