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Benefit Plans - Other Post-Retirement Benefits (Details)
$ in Thousands
 6 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2015
USD ($)
age
Schedule of Defined Benefit Plans and Defined Contribution Plans Disclosures [Line Items]      
Post-retirement medical, maximum age of coverage | age     65
Post-retirement medical, minimum service requirement     5 years
Post-retirement medical, minimum age of coverage | age     55
Post-retirement medical, post 55, pre 2006 employee percentage of monthly insurance     20.00%
Post-retirement medical, post 55, post 2006 employee percentage of monthly insurance     100.00%
Post-retirement medical, post 2006 reduction in premium service requirement     10 years
Post-retirement medical, post 55 with post 2006 reduction in premium service requirement percentage of monthly insurance     50.00%
Weighted-average discount rates used to determine the benefit obligations 4.20% 3.65%  
Weighted average discount rate use to determine net periodic benefit costs 3.80% 4.20% 3.50%
Postretirement Medical Plan      
Schedule of Defined Benefit Plans and Defined Contribution Plans Disclosures [Line Items]      
Plan termination | $     $ 6,632
Defined benefit gain on plan termination | $     $ (5,600)