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Postretirement Benefit Obligations - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Retirement Benefits [Abstract]    
Service costs $ 2,001 $ 1,908
Interest costs 469 333
Expected return on plan assets (287) (76)
Net periodic benefit cost $ 2,183 $ 2,165