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Significant accounting policies - New Accounting Standards Issued But Not Yet Effective (Details)
$ in Millions
Dec. 31, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
item
Significant accounting policies    
Number of equity investments accounted for at cost | item 1 1
Carrying value | $ $ 4.1 $ 3.8
Number of loan receivable carried at amortized cost | item 1  
Loan receivable at amortized cost | $ $ 30.1