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Stock Based Compensation Non Cash Stock Based Compensation Expense (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block] The non-cash stock-based compensation expense for the years ended December 31 is as follows:
(in thousands)
 
2017
 
2016
 
2015
Stock Incentive Plan
 
$
24,899

 
$
11,049

 
$
11,111

Employee Stock Purchase Plan
 
4,025

 
3,698

 
3,430

Performance Stock Plan
 
1,707

 
1,305

 
972

Incentive Stock Option Plan
 
—

 
—

 
—

Total
 
$
30,631

 
$
16,052

 
$
15,513

    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation $ 30,631 $ 16,052 $ 15,513
Stock Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation 24,899 11,049 11,111
PerformanceStock​Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation 1,707 1,305 972
Incentive Stock Option Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation 0 0 0
Employee Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation $ 4,025 $ 3,698 $ 3,430