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Share-Based Compensation and Capital Stock Summary Of Activity Related To Restricted Stock Grants (Details) - Restricted stock - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Share-Based Compensation and Capital Stock [Line Items]        
Restricted share grants (in shares) 21,562 21,357 167,142 188,509
Weighted average grant date fair value per share (in dollars per share) $ 62.34 $ 48.03 $ 61.19 $ 51.21
Aggregate grant date fair value ($000) $ 1,344 $ 1,026 $ 10,227 $ 9,654
Restricted shares forfeited (in shares) 331 14,072 23,599 23,686
Fair value of shares vested ($000) $ 2,766 $ 180 $ 10,677 $ 7,363
Minimum        
Share-Based Compensation and Capital Stock [Line Items]        
Vesting service period of shares granted 24 months 12 months 36 months 12 months
Maximum        
Share-Based Compensation and Capital Stock [Line Items]        
Vesting service period of shares granted 48 months 48 months 48 months 48 months