Annual report pursuant to Section 13 and 15(d)

Employee Retirement Plans - Assumptions (Details)

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Employee Retirement Plans - Assumptions (Details)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension plan          
Assumptions used in determining net periodic pension cost for the plans          
Discount rates (as a percent)     4.20%    
Expected long-term rate of return on assets (as a percent)     6.50% 7.25% 7.25%
Pension plan | Minimum          
Assumptions used in determining net periodic pension cost for the plans          
Discount rates (as a percent)       4.50% 3.90%
Pension plan | Maximum          
Assumptions used in determining net periodic pension cost for the plans          
Discount rates (as a percent)         4.00%
Salaried Pension Plan          
Assumptions used in determining net periodic pension cost for the plans          
Rates of increase in compensation levels: Salaried (as a percent)     3.50% 3.50% 3.50%
Assumptions used to determine the benefit obligation          
Discount rate (as a percent)     4.20%    
Salaried Pension Plan | Forecast          
Assumptions used in determining net periodic pension cost for the plans          
Expected long-term rate of return on assets (as a percent) 5.80%        
Hourly Pension Plan          
Assumptions used to determine the benefit obligation          
Discount rate (as a percent)   3.60% 4.50%    
Hourly Pension Plan | Forecast          
Assumptions used in determining net periodic pension cost for the plans          
Expected long-term rate of return on assets (as a percent) 6.50%        
Other postretirement benefit cost          
Assumptions used in determining net periodic pension cost for the plans          
Discount rates (as a percent)   3.80% 4.10% 3.70%  
Summary of healthcare cost trend rates          
Ultimate healthcare cost trend rate (as a percent)   4.50% 4.50% 4.50%  
Participation (as a percent)   60.00% 60.00% 60.00%  
Assumed health care cost trend rate for the next fiscal year (as a percent)   7.00% 7.00%    
Ultimate rate in 2025 (as a percent)   4.50%      
Ultimate rate in 2024 (as a percent)     4.50%    
Ultimate rate in 2023 (as a percent)       4.50%  
Assumptions used to determine the benefit obligation          
Discount rate (as a percent)   4.10% 3.70%    
Other postretirement benefit cost | Under 65          
Summary of healthcare cost trend rates          
Assumed health care cost trend rate for the next fiscal year (as a percent)       7.00%