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Segment reporting (Details Text) (Detail)
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting Details Text [Abstract]          
Revenue arising, customers located abroad 2.00% 2.00% 2.00% 5.00% 8.00%