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Equity Compensation and Employee Benefit Plans (Narrative) (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Equity Compensation [Abstract]      
Maximum shares of common stock | shares 20,000,000    
Maximum annual shares awarded to employee | shares 500,000    
Shares available for granting additional awards | shares 7,349,879    
Unrecognized compensation cost of non-vested equity compensation awards | $ $ 6,600,000    
Unrecognized compensation cost of non-vested equity compensation awards expected to be recognized over a weighted average period (years) 2 years 1 month 6 days    
Maximum annual stock purchase plan contributions by plan participants | $ $ 20,000    
Percentage of company matching contribution to employee stock purchase plan 15.00%    
Percentage of interest accrues on purchase plan contributions 5.25%    
Number of participants in executive nonqualified excess plan 41    
Number of options granted | shares 0 0 0
Stock Options      
Equity Compensation [Abstract]      
Maximum period for exercisable options (years and days) 10 years 1 day    
Stock Options | Minimum      
Equity Compensation [Abstract]      
Vesting period (months) 24 months    
Stock Options | Maximum      
Equity Compensation [Abstract]      
Vesting period (months) 72 months    
Restricted Stock      
Equity Compensation [Abstract]      
Fair value of awards vested | $ $ 4,400,000 $ 4,300,000 $ 4,500,000
Restricted Stock | Minimum      
Equity Compensation [Abstract]      
Vesting period (months) 12 months    
Restricted Stock | Maximum      
Equity Compensation [Abstract]      
Vesting period (months) 60 months    
Performance Shares      
Equity Compensation [Abstract]      
Fair value of awards vested | $ $ 1,000,000 $ 1,600,000 $ 1,100,000
Performance Shares | Minimum      
Equity Compensation [Abstract]      
Vesting period (months) 12 months    
Performance Shares | Maximum      
Equity Compensation [Abstract]      
Vesting period (months) 60 months