Summary of significant accounting policies - Intangible assets and goodwill (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020
Significant accounting policies
Insurance recoveries received	$ 183
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		2 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		2 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		1 year
Trade name and other | Maximum
Significant accounting policies
Estimated useful life		20 years
Non-compete agreements
Significant accounting policies
Estimated useful life		2 years