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Share-Based Compensation and Capital Stock Summary Of Activity Related To PVRSU Grants (Details) - Performance Vested Restricted Stock Units - 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]        
Performance vested restricted stock units granted at target (in shares) 3,545 10,387 162,523 89,944
Weighted average grant date fair value per share (in dollars per share) $ 62.00 $ 48.14 $ 60.63 $ 47.85
Aggregate grant date fair value ($000) $ 220 $ 500 $ 9,853 $ 4,304
Stock units forfeited (in shares) 331 26,363 72,117 54,556
Requisite service period 30 months      
Minimum        
Share-Based Compensation and Capital Stock [Line Items]        
Requisite service period   30 months 9 months 9 months
Maximum        
Share-Based Compensation and Capital Stock [Line Items]        
Requisite service period 30 months 36 months 36 months 43 months