Annual report pursuant to Section 13 and 15(d)

Employee Retirement Plans - Asset Allocation (Details)

v3.20.4
Employee Retirement Plans - Asset Allocation (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Pension plan    
Employee retirement plans    
Actual allocation of pension plan assets $ 38,053  
Qualified Hourly Pension Plan [Member]    
Employee retirement plans    
Target (as a percent) 100.00%  
Actual allocation of pension plan assets   $ 7,495
Weighted-average asset allocation (as a percent)   100.00%
Qualified Hourly Pension Plan [Member] | Large Cap Equity    
Employee retirement plans    
Target (as a percent) 5.00%  
Weighted-average asset allocation (as a percent) 0.00%  
Qualified Hourly Pension Plan [Member] | Mid Cap Equity    
Employee retirement plans    
Target (as a percent) 0.00%  
Weighted-average asset allocation (as a percent) 0.00%  
Qualified Hourly Pension Plan [Member] | Small Cap Equity    
Employee retirement plans    
Target (as a percent) 0.00%  
Weighted-average asset allocation (as a percent) 0.00%  
Qualified Hourly Pension Plan [Member] | International Equity    
Employee retirement plans    
Target (as a percent) 2.00%  
Weighted-average asset allocation (as a percent) 0.00%  
Qualified Hourly Pension Plan [Member] | Emerging markets Equity    
Employee retirement plans    
Target (as a percent) 0.00%  
Weighted-average asset allocation (as a percent) 0.00%  
Qualified Hourly Pension Plan [Member] | Fixed Income and Cash Equivalents    
Employee retirement plans    
Target (as a percent) 90.00%  
Actual allocation of pension plan assets $ 7,388  
Weighted-average asset allocation (as a percent) 99.00%  
Qualified Hourly Pension Plan [Member] | Real Estate    
Employee retirement plans    
Target (as a percent) 3.00%  
Actual allocation of pension plan assets $ 107  
Weighted-average asset allocation (as a percent) 1.00%  
Salaried Pension Plan    
Employee retirement plans    
Target (as a percent)   100.00%
Actual allocation of pension plan assets   $ 30,558
Weighted-average asset allocation (as a percent)   100.00%
Salaried Pension Plan | Large Cap Equity    
Employee retirement plans    
Target (as a percent) 5.00%  
Weighted-average asset allocation (as a percent) 0.00%  
Salaried Pension Plan | Mid Cap Equity    
Employee retirement plans    
Target (as a percent) 0.00%  
Weighted-average asset allocation (as a percent) 0.00%  
Salaried Pension Plan | Small Cap Equity    
Employee retirement plans    
Target (as a percent) 0.00%  
Weighted-average asset allocation (as a percent) 0.00%  
Salaried Pension Plan | International Equity    
Employee retirement plans    
Target (as a percent) 2.00%  
Weighted-average asset allocation (as a percent) 0.00%  
Salaried Pension Plan | Emerging markets Equity    
Employee retirement plans    
Target (as a percent) 0.00%  
Weighted-average asset allocation (as a percent) 0.00%  
Salaried Pension Plan | Fixed Income and Cash Equivalents    
Employee retirement plans    
Target (as a percent) 90.00%  
Actual allocation of pension plan assets $ 30,009  
Weighted-average asset allocation (as a percent) 98.00%  
Salaried Pension Plan | Real Estate    
Employee retirement plans    
Target (as a percent) 3.00%  
Actual allocation of pension plan assets $ 549  
Weighted-average asset allocation (as a percent) 2.00%