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Weighted average assumptions (Details 4) - Qualified Plan
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Underfunded Plan | Pension Benefits    
Used to determine benefit obligation    
Discount rate 5.06% 2.64%
Expected return on plan assets 6.50% 6.00%
Used to determine benefit cost    
Discount rate 2.64% 2.30%
Expected return on plan assets 6.00% 6.00%
Overfunded Plan | Other Postretirement Benefits    
Used to determine benefit obligation    
Discount rate 5.07% 2.66%
Expected return on plan assets 6.00% 5.50%
Rate of compensation increase 3.00% 3.00%
Used to determine benefit cost    
Discount rate 2.66% 2.30%
Expected return on plan assets 5.50% 5.50%
Rate of compensation increase 3.00% 3.00%