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Employee benefit plans - Schedule of Pension Plan Weighted-Average Asset Allocations (Details)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations 100.00% 100.00%
Cash and Cash Equivalents [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations 5.90% 2.40%
Equity Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations 57.00% 59.30%
Corporate Debt Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations 23.00% 14.90%
U.S. Government Agencies [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations 14.10% 23.40%