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Employee benefit plans - Components of Net Periodic Pension Expense (Income) and Other Amounts Recognized in Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):      
Net loss (gain) $ 6,628 $ 6,946  
Pension Benefits      
Net periodic pension cost:      
Interest cost 650 665 $ 696
Expected return on plan assets (915) (893) (931)
Amortization of unrecognized net gain 195 209 198
Net periodic pension cost (70) (19) (37)
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):      
Net loss (gain) 318 917 296
Deferred tax (67) (321) (104)
Total recognized in other comprehensive income 251 596 192
Total recognized in net period pension cost and other comprehensive income 181 $ 577 $ 155
Estimated net loss for defined benefit pension plan that will be amortized from accumulated other comprehensive loss into net periodic benefit cost over next fiscal year $ 19