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Postretirement Benefit Obligations - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Retirement Benefits [Abstract]        
Service costs $ 1,908   $ 3,816  
Interest costs 333   666  
Expected return on plan assets (76)   (152)  
Other   $ 1,717   $ 3,434
Net periodic benefit cost $ 2,165 1,717 $ 4,330 3,434
Net periodic benefit cost related to participation in Shared Plan   $ 1,700   $ 3,400