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Stock Plans (Summary of Restricted Stock Award Activity) (Details)
shares in Thousands
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Restricted Stock Units (RSUs) [Member]  
Restricted stock award activity  
Performance factor percentage minimum 0.00%
Performance factor percentage maximum 200.00%
Restricted Stock Awards [Member]  
Restricted stock award activity  
Nonvested at beginning of year | shares 521
Shares, granted | shares 231
Shares, vested | shares (229)
Shares, forfeited | shares (27)
Nonvested at end of year | shares 496
Weighted-average grant-date fair value, nonvested at beginning of year (per share) | $ / shares $ 46.48
Weighted-average grant-date fair value of stock granted (per share) | $ / shares 54.75
Weighted-average grant-date fair value, vested (per share) | $ / shares 46.11
Weighted-average grant-date fair value, forfeited (per share) | $ / shares 50.12
Weighted-average grant-date fair value, nonvested at end of year (per share) | $ / shares $ 50.30