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Benefit Plans - Key Assumptions for Projected Benefit Obligation and Net Periodic Benefit Cost (Details)
6 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2015
Projected benefit obligation:      
Discount rate 4.20% 3.65%  
Rate of compensation increase 4.30% 3.00%  
Net periodic benefit costs:      
Discount rate 3.80% 4.20% 3.50%
Expected long-term rate of return 7.00% 6.25%  
Rate of compensation increase 4.03% 4.30%