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Borrowings and debt (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of short term borrowings [Line Items]    
Average outstanding balance during the year $ 710,021 $ 1,348,230
Maximum outstanding balance at any month - end 1,072,723 1,876,322
Total short-term borrowings and debt 1,072,723 1,470,075
Short-term borrowings [member]    
Disclosure of short term borrowings [Line Items]    
Total short-term borrowings and debt 1,062,223 1,445,075
Short term debt [Member]    
Disclosure of short term borrowings [Line Items]    
Total short-term borrowings and debt 10,500 25,000
US dollar [Member]    
Disclosure of short term borrowings [Line Items]    
Total short-term borrowings and debt 1,044,500 1,470,000
Mexican peso [Member]    
Disclosure of short term borrowings [Line Items]    
Total short-term borrowings and debt 28,223 75
Fixed interest rate [member] | Short-term borrowings [member]    
Disclosure of short term borrowings [Line Items]    
Total short-term borrowings and debt 429,069 788,075
Fixed interest rate [member] | Short term debt [Member]    
Disclosure of short term borrowings [Line Items]    
Total short-term borrowings and debt $ 10,500 $ 25,000
Fixed interest rate [member] | Short term borrowings and debt [Member] | Mexican peso [Member]    
Disclosure of short term borrowings [Line Items]    
Borrowings, interest rate 7.92% 6.16%
Floating interest rate [member] | Short-term borrowings [member]    
Disclosure of short term borrowings [Line Items]    
Total short-term borrowings and debt $ 633,154 $ 657,000
Floating interest rate [member] | Short term debt [Member]    
Disclosure of short term borrowings [Line Items]    
Total short-term borrowings and debt $ 0 $ 0
Floating interest rate [member] | Short term borrowings and debt [Member] | Mexican peso [Member]    
Disclosure of short term borrowings [Line Items]    
Borrowings, interest rate   5.72%
Bottom of range [member] | Fixed interest rate [member] | Short term borrowings and debt [Member] | US dollar [Member]    
Disclosure of short term borrowings [Line Items]    
Borrowings, interest rate 1.60% 1.10%
Bottom of range [member] | Floating interest rate [member] | Short term borrowings and debt [Member] | US dollar [Member]    
Disclosure of short term borrowings [Line Items]    
Borrowings, interest rate 1.77% 1.14%
Bottom of range [member] | Floating interest rate [member] | Short term borrowings and debt [Member] | Mexican peso [Member]    
Disclosure of short term borrowings [Line Items]    
Borrowings, interest rate 7.68%  
Weighted Average [Member] | Short term borrowings and debt [Member]    
Disclosure of short term borrowings [Line Items]    
Borrowings, interest rate 2.16% 1.30%
Weighted Average [Member] | During the Year [Member] | Short term borrowings and debt [Member]    
Disclosure of short term borrowings [Line Items]    
Borrowings, interest rate 1.66% 1.10%
Top of range [member] | Fixed interest rate [member] | Short term borrowings and debt [Member] | US dollar [Member]    
Disclosure of short term borrowings [Line Items]    
Borrowings, interest rate 1.95% 1.50%
Top of range [member] | Floating interest rate [member] | Short term borrowings and debt [Member] | US dollar [Member]    
Disclosure of short term borrowings [Line Items]    
Borrowings, interest rate 2.08% 1.48%
Top of range [member] | Floating interest rate [member] | Short term borrowings and debt [Member] | Mexican peso [Member]    
Disclosure of short term borrowings [Line Items]    
Borrowings, interest rate 7.89%