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Postretirement Benefit Cost - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Retirement Benefits [Abstract]        
Service costs $ 1,714 $ 2,001 $ 3,427 $ 4,003
Interest costs 521 469 1,042 938
Expected return on plan assets (334) (287) (668) (575)
Net periodic benefit cost $ 1,901 $ 2,183 $ 3,801 $ 4,366