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Stock-Based Compensation - Plan Summary (Narrative) (Details)
3 Months Ended 12 Months Ended
Mar. 31, 2008
shares
Dec. 31, 2017
compensation_plan
Jun. 27, 2017
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shareholder-approved stock-based compensation plans | compensation_plan   2  
2008 EIP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Reserved for issuance (shares) 10,600,000    
2017 EICP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Reserved for issuance (shares)     3,400,000
Available for future grant (shares)     3,100,000
Employees      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period   3 years  
Directors      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period   1 year  
Options | 2008 EIP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award expiration period 7 years    
Options | Employees | 2008 EIP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Options | Directors | 2008 EIP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 1 year