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Stock-Based Incentive Plan (Compensation Expense) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense $ 7,453 $ 4,386 $ 12,150 $ 8,230
Share-based compensation arrangement, compensation cost 3,400   3,900  
Service-based stock appreciation rights (SARs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense 2,675 1,731 4,023 3,331
Service-based restricted stock units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense 2,634 2,114 4,790 4,318
Market performance-based restricted stock units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense 959 170 $ 1,305 174
Share-based compensation arrangement, vesting period     3 years  
Operating performance-based restricted stock units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense $ 1,185 $ 371 $ 2,032 $ 407
Share-based compensation arrangement, vesting period     3 years  
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation arrangement, vesting period     4 years