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Incentive Plans (Summary of Changes in Stock Options Outstanding) (Details)
 12 Months Ended
Dec. 31, 2015
$ / shares
shares
Dec. 31, 2014
$ / shares
shares
Number of Shares    
Beginning balance | shares 660,635 621,149
Granted at FMV | shares 72,350 127,400
Exercised | shares (236,620) (74,766)
Forfeited | shares (30,609) (13,148)
Ending balance | shares 465,756 660,635
Average Exercise Price (in dollars per share)    
Beginning balance | $ / shares $ 36.00 $ 35.13
Granted at FMV | $ / shares 70.57 39.46
Exercised | $ / shares 33.80 34.21
Forfeited | $ / shares 40.21 38.36
Ending balance | $ / shares $ 42.22 $ 36.00
Exercisable (in shares) | shares 127,077 88,445
Shares exercisable, average exercise price (in dollars per share) | $ / shares $ 36.71 $ 34.54