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Lease obligations - Narrative (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2019
USD ($)
Disclosure of leases [Abstract]  
Expense relating to variable lease payments not included in measurement of lease liabilities $ 83,700
Expense relating to short-term leases for which recognition exemption has been used 500
Lease not yet commenced, amount 6,600
Potential future lease payments not included in lease liabilities (undiscounted) $ 70,588