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Weighted average assumptions (Details 5) - Qualified Plan
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Underfunded Plan | Pension Benefits    
Used to determine benefit obligation    
Discount rate 2.30% 2.96%
Expected return on plan assets 6.00% 6.25%
Used to determine benefit cost    
Discount rate 2.96% 4.03%
Expected return on plan assets 6.25% 6.25%
Overfunded Plan | Other Postretirement Benefits    
Used to determine benefit obligation    
Discount rate 2.30% 3.00%
Expected return on plan assets 5.50% 5.75%
Rate of compensation increase 3.00% 3.00%
Used to determine benefit cost    
Discount rate 3.00% 4.05%
Expected return on plan assets 5.75% 5.75%
Rate of compensation increase 3.00% 3.00%