Summary of significant accounting policies - Property, plant and equipment (Details)	12 Months Ended
	Dec. 31, 2017
Buildings
Property, plant and equipment
Residual value, percentage	10.00%
Buildings | Maximum
Property, plant and equipment
Useful life (in years)	60 years
Trains | Maximum
Property, plant and equipment
Useful life (in years)	75 years
River cruise ship and canal boats
Property, plant and equipment
Useful life (in years)	25 years
Furniture, fixtures and equipment | Maximum
Property, plant and equipment
Useful life (in years)	25 years
Furniture, fixtures and equipment | Minimum
Property, plant and equipment
Useful life (in years)	3 years