Accounting policies (Details)	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Threshold percentage for recognizing actuarial gains and losses	10.00%
Overhauls of drilling units
Property, Plant and Equipment [Line Items]
Estimated economic useful life	5 years
Drilling units
Property, Plant and Equipment [Line Items]
Estimated economic useful life	30 years
Period within which management is actively committed to a probable sale of assets classified as held for sale	1 year
Equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated economic useful life	3 years
Equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated economic useful life	5 years