Summary of significant accounting policies - Property and equipment (Details)	12 Months Ended
	Dec. 31, 2018
Software | Minimum
Property and equipment
Estimated useful lives	2 years
Software | Maximum
Property and equipment
Estimated useful lives	5 years
Computer equipment | Minimum
Property and equipment
Estimated useful lives	3 years
Computer equipment | Maximum
Property and equipment
Estimated useful lives	5 years
Furniture, fixtures and office equipment | Minimum
Property and equipment
Estimated useful lives	3 years
Furniture, fixtures and office equipment | Maximum
Property and equipment
Estimated useful lives	5 years
Leasehold improvements | Minimum
Property and equipment
Estimated useful lives	2 years
Leasehold improvements | Maximum
Property and equipment
Estimated useful lives	18 years