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Benefit Plans, Weighted Average Assumptions Used (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Weighted average assumptions used to determine net periodic benefit cost [Abstract]      
Discount rate 3.56% 3.88% 3.71%
Expected return on plan assets 6.61% 6.89% 6.89%
Rate of compensation increase 3.01% 3.03% 3.04%
Weighted average assumptions used to determine benefit obligation [Abstract]      
Discount rate 3.16% 3.60% 3.89%
Rate of compensation increase 3.01% 2.96% 3.04%
Actual return on pension assets 11.00% 8.00% 1.00%
Maximum health care cost trend rate 5.00%