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Stock Plans - Stock Option Activity (Details) - Stock option Awards - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares      
Balance at the beginning of the period (in shares) 1,576,000 2,064,000 2,391,000
Granted (in shares) 0 0 17,000
Exercised (in shares) (18,000) (194,000) (192,000)
Expired or forfeited (in shares) (164,000) (294,000) (152,000)
Balance at the end of the period (in shares) 1,394,000 1,576,000 2,064,000
Weighted Average Exercise price      
Balance at the beginning of the period (in dollars per share) $ 35.18 $ 32.91 $ 31.65
Granted (in dollars per share)     30.22
Exercised (in dollars per share) 30.03 12.18 12.95
Expired or forfeited (in dollars per share) 37.47 34.44 38.15
Balance at the end of the period (in dollars per share) $ 34.97 $ 35.18 $ 32.91