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Benefit Plans - Changes in Benefit Obligations (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2015
Change in benefit obligation:      
Benefit obligation at the beginning of year $ 34,319 $ 28,914  
Service cost 668 614  
Interest cost 598 1,192  
Plan amendments 3,067 0  
Actuarial loss (gain) 2,436 (1,091)  
Projected benefit obligation at end of year $ 28,914 $ 30,877  
Postretirement Medical Plan      
Change in benefit obligation:      
Benefit obligation at the beginning of year     $ 5,414
Service cost     370
Interest cost     212
Plan amendments     0
Plan termination     (6,632)
Actuarial loss (gain)     636
Projected benefit obligation at end of year     $ 0