Annual report pursuant to Section 13 and 15(d)

Retirement Plans and Related Post Retirement Benefits - Narrative (Details)

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Retirement Plans and Related Post Retirement Benefits - Narrative (Details)
 12 Months Ended
Dec. 31, 2018
USD ($)
retirement_plan
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Defined Benefit Plan Disclosure      
Number of non-qualified supplemental retirement defined benefit plans | retirement_plan 2    
Accumulated benefit obligation of the plans $ 21,000,000.0 $ 22,700,000  
Fair value of plan assets at period end 0 0  
Unrecognized prior service costs, net 3,100,000    
Unrecognized prior service costs, tax 600,000    
Unrecognized actuarial losses 3,200,000    
Unrecognized actuarial losses, net 4,100,000    
Unrecognized actuarial losses, tax 900,000    
Net periodic pension cost 300,000    
Net periodic pension cost net of tax 400,000    
Net periodic pension cost, tax 100,000    
Actuarial loss 200,000    
Actuarial loss, net of tax 300,000    
Actuarial loss, tax 100,000    
SERP      
Defined Benefit Plan Disclosure      
Unrecognized prior service costs 2,500,000    
Current accrued pension liability 400,000    
Long-term accrued pension liability 21,600,000    
Expected future payments in 2019 (less than for SERP Medical) 300,000    
Expected future payments in 2020 (less than for SERP Medical) 300,000    
Expected future payments in 2021 (less than for SERP Medical) 300,000    
Expected future payments in 2022 (less than for SERP Medical) 300,000    
Expected future payments in 2023 (less than for SERP Medical) 300,000    
Benefits to be paid in the aggregate for the following five years 4,100,000    
SERP Medical      
Defined Benefit Plan Disclosure      
Fair value of plan assets at period end 100,000    
Unrecognized actuarial losses 400,000    
Unrecognized actuarial losses, tax 100,000    
Current accrued pension liability 100,000    
Long-term accrued pension liability 1,100,000    
Expected future payments in 2019 (less than for SERP Medical) 100,000    
Expected future payments in 2020 (less than for SERP Medical) 100,000    
Expected future payments in 2021 (less than for SERP Medical) 100,000    
Expected future payments in 2022 (less than for SERP Medical) 100,000    
Expected future payments in 2023 (less than for SERP Medical) 100,000    
Benefits to be paid in the aggregate for the following five years $ 400,000    
Annual increase in the cost of health care benefits 5.20%    
Change in retirement benefit obligation $ 100,000    
Percentage of fund 91.90%    
Contributions of employer $ 1,100,000 $ 1,100,000 $ 1,100,000
Total employer contribution 1.00%    
SERP Medical | Minimum      
Defined Benefit Plan Disclosure      
Annual increase in the cost of health care benefits range 4.30%    
SERP Medical | Maximum      
Defined Benefit Plan Disclosure      
Annual increase in the cost of health care benefits range 6.10%