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Retirement plans - Expense recognized in income statement (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Net defined benefit pension plan expense:    
Current service cost $ 2,639 $ 1,981
Past service cost 0 1,279
Net interest cost 800 827
Net defined benefit pension plan expense $ 3,439 $ 4,087