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Postretirement Benefit Obligations - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Retirement Benefits [Abstract]        
Service costs $ 2,001 $ 1,908 $ 6,004 $ 5,724
Interest costs 469 333 1,407 999
Expected return on plan assets (287) (76) (862) (228)
Net periodic benefit cost $ 2,183 $ 2,165 $ 6,549 $ 6,495