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Employee benefit plans - Schedule of Pension Plan Weighted-Average Asset Allocations (Details) - Pension Benefits
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure    
Weighted-average asset allocations 100.00% 100.00%
Cash and cash equivalents    
Defined Benefit Plan Disclosure    
Weighted-average asset allocations 6.20% 5.00%
Equity securities    
Defined Benefit Plan Disclosure    
Weighted-average asset allocations 61.90% 56.20%
Corporate bonds    
Defined Benefit Plan Disclosure    
Weighted-average asset allocations 20.60% 23.60%
U.S. Treasuries and Government agencies    
Defined Benefit Plan Disclosure    
Weighted-average asset allocations 11.30% 15.20%