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Investment property - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about investment property [line items]      
Investment property $ 34,295 $ 40,725 $ 42,342
Fair value      
Disclosure of detailed information about investment property [line items]      
Investment property 34,295 40,725  
Level 3 | Fair value      
Disclosure of detailed information about investment property [line items]      
Investment property 34,200 40,700  
Increase (decrease) in investment property $ 3,400 $ 4,100