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Employee benefit plans - Schedule of Weighted-Average Assumptions Used to Determine Benefit Obligations and Related Expenses (Details) - Pension Benefits
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure      
Discount rate, Obligation 4.00% 4.00% 4.00%
Discount rate, Expense 3.75% 4.00% 4.00%
Expected long-term return on plan assets 7.50% 7.50% 7.50%