Significant accounting policies - Intangible assets and goodwill (Details)	12 Months Ended
	Dec. 31, 2018
Customer relationships | Minimum
Significant accounting policies
Estimated useful life	9 years
Customer relationships | Maximum
Significant accounting policies
Estimated useful life	21 years
Backlog | Minimum
Significant accounting policies
Estimated useful life	3 years
Backlog | Maximum
Significant accounting policies
Estimated useful life	5 years
Technologies | Minimum
Significant accounting policies
Estimated useful life	5 years
Technologies | Maximum
Significant accounting policies
Estimated useful life	13 years
Software | Minimum
Significant accounting policies
Estimated useful life	3 years
Software | Maximum
Significant accounting policies
Estimated useful life	10 years
Image library
Significant accounting policies
Estimated useful life	5 years
Trade name and other | Minimum
Significant accounting policies
Estimated useful life	5 years
Trade name and other | Maximum
Significant accounting policies
Estimated useful life	20 years
Non-compete agreements
Significant accounting policies
Estimated useful life	2 years