Annual report pursuant to Section 13 and 15(d)

Retirement and Long-Term Incentive Plans - Narrative (Details)

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Retirement and Long-Term Incentive Plans - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Employer discretionary contribution $ 0 $ 670,000 $ 2,300,000
Available shares for future issuance (in shares) 833,443    
Total unrecognized compensation costs $ 2,700,000    
Recognition of compensation cost, weighted average period 1 year 7 months 6 days    
Total fair value of shares vested $ 2,100,000    
Share-based compensation cost charged against income 2,741,000 3,125,000 2,707,000
Total income tax (expense) benefit under share-base compensation (253,000) 0 0
Incentive Plans      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation cost charged against income $ 2,700,000 2,100,000 2,700,000
Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares authorized (in shares) 1,000,000    
Employee Stock Option | Long Term Incentive Plan 2002      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares authorized (in shares) 500,000    
Employee Stock Option | Long Term Incentive Plan 2011      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares authorized (in shares) 500,000    
Employee Stock Option | Long Term Incentive Plan 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares authorized (in shares) 1,000,000    
Non Performance Based      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock vesting period, minimum 3 years    
Restricted Stock | Non-employee directors      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock vesting period 6 months    
Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance based share compensation expense $ 1,500,000 1,300,000 $ 1,100,000
Fair value of performance based shares granted $ 4,700,000 $ 1,600,000  
Maximum | Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options available for grant to an individual (more than) (in shares) 400,000    
Maximum | Employee Stock Option | Long Term Incentive Plan 2002      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options available for grant to an individual (more than) (in shares) 200,000    
Maximum | Employee Stock Option | Long Term Incentive Plan 2011      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options available for grant to an individual (more than) (in shares) 200,000    
Maximum | Employee Stock Option | Long Term Incentive Plan 2015      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options available for grant to an individual (more than) (in shares) 200,000    
Options available for grant to an outside director (more than) (in shares) 25,000    
2015 Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance period awards are earned     2 years
2016 Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance period awards are earned   3 years