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Retirement plans - Expense recognized in income statement (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Net defined benefit pension plan expense:    
Current service cost $ 1,879 $ 1,677
Past service cost 812 0
Net interest cost 720 715
Cost of settlement 0 0
Net defined benefit pension plan expense $ 3,411 $ 2,392