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Weighted average assumptions (Details 5) - Underfunded Plan [Member] - Qualified Plan [Member]
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension Benefits    
Used to determine benefit obligation    
Discount rate 3.38% 3.83%
Expected return on plan assets 6.75% 6.75%
Used to determine benefit cost    
Discount rate 3.83% 4.00%
Expected return on plan assets 6.75% 6.75%
Other Postretirement Benefits    
Used to determine benefit obligation    
Discount rate 3.41% 3.86%
Expected return on plan assets 5.75% 5.75%
Rate of compensation increase 3.00% 3.00%
Used to determine benefit cost    
Discount rate 3.86% 4.06%
Expected return on plan assets 5.75% 5.75%
Rate of compensation increase 3.00% 3.00%